Why is the word “all” spelled this way instead of “aal”?

Billy Kerr’s answer to Why is the word “all” spelled this way instead of “aal”? is right, but lemme add a bit to it.

While English spelling looks pretty random, there is a predictability to it if you assume that it used to make sense in Middle English. So through a particular vowel change in Middle English, a before l tends to be pronounced as /ɑː/ or /ɔː/, depending on dialect. walk, call, paltry, etc. And the presence of the l is a cue to us that the a will do that. (Unless we know the word isn’t English, of course, as in palimony or banal.)

Now. You speak a version of English that has enough distance from standard English, both in terms of national identity and language structure, that it might as well be a separate language. Which means you get to spell it differently than English. And in that language, the l after a drops off. How do you spell all now?

I speak, of course, of Scots.

You could keep spelling it as all, and introduce a rule that l(l) after a is silent. It’s not like English hasn’t done things like that. But:

• It’s hard enough to get people to write and read in Scots to begin with; you don’t want to introduce more crazy spelling rules.
• Part of the point of a Scots spelling is that it looks different from English, to people who already know how to write in English. You show them <all>, they’ll just pronounce it as /ɑːlˠ/.

You could start with all, and then put in an apostrophe to indicate that the l has fallen off: a’ . Because the apostrophe means an l has fallen off, you know that the a’ will be pronounced as /ɑː/.

That’s what Robbie Burns did. The problem with that though, is that it takes Middle English as a starting point: you’re writing an apostrophe because there used to be a letter there in Middle English. And there still is a letter there in Modern English. Which means your spelling system is derived from English a little too obviously. No wonder that convention is now called the Apologetic apostrophe.

You could decide that the English spelling all can get stuffed,  and you’ll spell the word phonetically—using the closest spelling English orthography allows you. That gives you aw. Which I believe is the current Modern Scots recommendation, and is also what Scots was written like in its literary heyday.

OR: In between Robbie Burns and now, you could say that the Sassenach can be buggered, and you’ll invent a brand new spelling for a brand new language, that owes nothing to those Southern fiends, and that doesn’t expect you to know that an l used to be there. And if your new spelling looks a bit more Viking than English, so much the better.

Hence: aa.

And if you retrofit Scots spelling onto English all, you get aal.

Which I suspect is what OP is doing. I don’t see where else he’d have gotten <aa> from: <aa> is not otherwise used in English for anything.