Tolerance - How significant is cross-tolerance

[Tortoise]

I was speaking to a friend who has an unusual gastric/abdominal condition that causes moderate levels of pain. She mentioned that she is routinely prescribed tramadol alternating with panadeine forte (30mg codeine x2). She typically uses the tramadol for a few weeks until she notices her tolerance has built up and then switches to codeine, which she uses for a few weeks until her tolerance to that builds up. She then swaps back to the tramadol and claims that her tolerance to has reduced significantly in the time that she has been taking the codeine.

I find this a little odd, as I thought cross-tolerance to opiates occurred across the board. Is tramadol special, or do all opiates not exhibit 100% cross tolerance. As an example, if one built a tolerance to hydromorphone, would they not also be building a very significant tolerance to oxcodone?