[datr]

Maybe I'm assuming wrong but I guess in your example zone A and B are the same and that the different zone names users see don't represent different ways of spreading resources. If so, why aren't they named consistently? If not, are there any details on how and why they've set up their zones (or am I overlooking another assumption I've made?)

[saurik]

Due to a quirk of both human nature and copy/paste example code, if the names of the availability zones were mapped the same for all users then 90% of requests would be to the zone named "A". To make certain that the zones get even usage, as they bring on new customers, they change the A-D order of the availability zones as seen by these new accounts.

[asdfaoeu]

If they weren't different it would likely everyone would just pick the same first zone, this way they can randomly assign resources keeping it even.

[dredmorbius]

Adding to the "why it's scrambled" comments: because data transfer pricing differs for intra- vs. inter- AZ traffic, if you need to coordinate with other entities to be in the same AZ, it's possible to do so.

But, yes, generally: Amazon randomly allocates AZ labels to given customers to avoid preferential clustering on any given first choice.