They randomise the AZ letters, because humans tend to shove things in 'a' before anywhere else - it's a psychological load-balancer.
If you want to figure out which of your AZs corresponds to another account's AZs, you can compare spot-prices, which are individual per AZ. Also, for some reason, my account doesn't have a 'b', just a-c-d-e. Weird.
Interesting that the idea is to distribute load across the zones (because people default to using A), but Google Cloud zones are uniform across all accounts and it seems to work for Google.
I heard that aws actually scrambled availability zones... so your 1a might be my 1c, etc.. Haven't confirmed it, but the comments seem to bear that out.