I think that, when talking about idempotency, there's the implicit assumption that the "rest of the world" stays the same while the sequence of operations is performed.

rfc2616 says:

> Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N > 0 identical requests is the same as for a single request.

https://datatracker.ietf.org/doc/html/rfc2616#page-51

Perhaps changes in the underlying data could be considered "expiration issues". Otherwise not even GET could be considered idempotent in many cases.