why is it better than AAKK? Is it more likely to make straights with AJ?

Of course I mean double suited in both cases

I think you mean AAJT

And if you think about it, the KK pair isn't used unless it hits a set and basically the AJT combination makes it "better" if you look at it this way:

AAKK=AA+KK+AK (three combinations, where the AK combo is very unlikely to make a straight)
AAJT=AA+AJ+AT+JT (four combinations with three straight combinations, one being very likely)

All-in preflop AAKK is a small favorite, but AAJT is much more flexible and plays better post flop due to fitting more flops better.

Maybe AATT is the best then. I only noticed AAKK did worse against random (= 100% range) than AAJJ every time I tried.

I find it hard to believe really, given they both make the same number of straights but AAKK will ALWAYS make top set if you hit a set (which isn't the case with AAJJ - i.e. KKQQ has a good chance of winning even if you flop a J).

I suppose one explanation might be that against a random hand drawn from the remaining 48 cards, JJ blocks more hands that have value against aces. That is, if you have AAJJ the JJ in your hand is interfering with (blocking) a wider range of wraps than the KK in AAKK. If this is the case, then I guess AATT should indeed be the nuts as it interferes with more straights (actually, maybe AA99 is better, or maybe there's a point at which the straight-interference value is counterbalanced by the possibility of your set losing to a higher one, and AATT is that point).

However, absolute showdown value isn't the be-all and end-all, so you'd rather have AAKK - AAKK on a KQ5 flop is a lot better than AAJJ on a QJ5 flop! AAKK plays better in a post-flop situation because you know if you set up it'll always be the top one.

Yeah it might not be of practical relevance. Interesting though that AAxx is that strong (xx= any pair). Also extremely strong in PLO8.

I rarely lay down AATT-AAKK PF.