The Aboleth Statblock states:

Int: 18 (+4)
Wis: 15 (+2)
Skills History +12, Perception +10
Senses passive Perception 20
CR: 10

A CR of 10 translates to a proficiency bonus of 4. This should make the bonuses +8 and +6 for History and Perception. However, the History and Perception bonuses seem to have 2x the proficiency bonus applied.

I'm fairly new to D&D, I'm trying to wrap my head around statblocks. I think the stat blocks may not be showing "expertise"?

Also, why would an Aboleth have expertise in History???

The Aboleth:
https://roll20.net/compendium/dnd5e/Aboleth

Aboleths do have [something which is mechanically equivalent to] Expertise in those skills

The Monster Manual uses the word 'expertise' when describing how certain creatures might have higher-than-expected bonuses to certain skills:

The Skills entry is reserved for monsters that are proficient in one or more skills. For example, a monster that is very perceptive and stealthy might have bonuses to Wisdom (Perception) and Dexterity (Stealth) checks.

A skill bonus is the sum of a monster's relevant ability modifier and its proficiency bonus, which is determined by the monster's challenge rating (as shown in the Proficiency Bonus by Challenge Rating table). Other modifiers might apply. For instance, a monster might have a larger-than-expected bonus (usually double its proficiency bonus) to account for its heightened expertise.

—Skills, Monster Manual, pg. 8

There's a lot of contention over whether this translates to such creatures literally having an unwritten feature, Expertise, in these skills (same as the Class Feature that Rogues and Bards have access to) or if this is just colloquial language (see here for more), but regardless, it's clear that monsters can and do have something which is mechanically equivalent to Expertise in certain skills.

As for why...

The Monster Manual entry for Aboleths has a lot to say on the subject, but one passage in particular stands out...

Before the coming of the gods, aboleths lurked in primordial oceans and underground lakes. They reached out with their minds and seized control of the burgeoning life-forms of the mortal realm, making those creatures their slaves. Their dominance made them like gods. Then the true gods appeared, smashing the aboleths' empire and freeing their slaves. Aboleths have never forgotten.

Aboleths have flawless memories. They pass on their knowledge and experience from generation to generation. Thus, the injury of their defeat by the gods remains perfectly preserved in their minds.

Aboleths' minds are treasure troves of ancient lore, recalling moments from prehistory with perfect clarity. They plot patiently and intricately across eons. Few creatures can conceive of the extent of an aboleth's plan.

—Aboleth, Monster Manual, pg. 14

So it is clear that Aboleths having enhanced Intellect, and in particular, enhanced memory, is well justified in the lore of the game, and is thus appropriately reflected in the game's mechanics.

Your math seems right, and an expertise in those skills would explain that difference.

However, PCs and anything else do not follow the same rules. Monsters, creatures, NPCs, etc. can all have different stats than a player character would have without having the additional features a player character would need to get those stats.

As for why an Aboleth would have expertise in history...

Aboleths, according to the Monster Manual, are older than the gods, and an aboleth remembers with perfect clarity everything it has ever experienced. Additionally, aboleths pass their memories down to their offspring, including their own parents' and ancestors' memories. So, every aboleth remembers perfectly everything it ever experienced, everything its parent had experienced up to when it was born, everything the parent aboleth's parent had experienced before the first one's parent was born, and so on, all the way back to the very first aboleth, who was old enough to remember a time before the gods existed!

I'd be surprised if they didn't have expertise in history after all that!

Incidentally, this also happens to be the reason every aboleth utterly hates and despises the gods, because every aboleth remembers, in perfect detail, when the gods overthrew their rule!

As per the Monster Manual, page 14...

Before the coming of the gods, aboleths lurked in primordial oceans and underground lakes. They reached out with their minds and seized control of the burgeoning life-forms of the mortal realm, making those creatures their slaves. Their dominance made them like gods. Then the true gods appeared, smashing the aboleths' empire and freeing their slaves. Aboleths have never forgotten.

Aboleths have flawless memories. They pass on their knowledge and experience from generation to generation. Thus, the injury of their defeat by the gods remains perfectly preserved in their minds.

Aboleths' minds are treasure troves of ancient lore, recalling moments from prehistory with perfect clarity. They plot patiently and intricately across eons. Few creatures can conceive of the extent of an aboleth's plan.