Objects that have no intrinsic order (like AssociativeArray) are enumerated in apparent random order
In the current implementation, the enumeration order depends on a combination of key hashes and order of item creation. It's unlikely to be in alphabetical sorted order for any non-trivial and non-contrived example.
I don't think your example of b,a,c would enumerate as a,b,c even, so I wonder if there is more to your use case than the example?
But again as Komag said, if you care about preserving arbitrary item order than using an ordered type such roArray or roList might serve better.