Winding arrangement in small toroidal mains isolation transformer
Possible Reason #1
The supplier may pre-wind the secondaries to suit common applications then, when they receive an order for a bunch of transformers having (say) a 220 volt input they wind the primary to suit the order. But equally, it could be done the other way round for very similar reasons.
Possible Reason #2
I expect it might be to avoid the much thinner primary wires being damaged when winding the much sturdier secondary wires over the primary.