There are several reasons why this could have been the case. As mentioned, there is not complete cross-tolerance with opiates, although often a lot amongst similar opiates. Also, the effects of hydrocodone and oxycodone are different. Some say they prefer hydrocodone as they say it is "dreamier", more sedating, and even more euphoric. Not all users feel this way, but some really do. There is also the possibility that you do metabolize hydrocodone in such a way that it provides you with a better effect than it would to most due to decreased or increased enzymes (I forget exactly how it works with hydrocodone). Personally, I have much more experience with oxycodone, but would explore with hydrocodone more if I had the chance. The only downside to hydrocodone over oxycodone that I really see is the fact that it doesn't last quite as long, and oxycodone seems just a bit more euphoric without as much sedation. I have also noticed hydrocodone to release more histamine than oxycodone, but wouldn't really mind that effect so long as I kept comfortably cool. These could all be potential upsides, however. I might want an opiate a bit more short-acting, that sedates me a bit more and makes me a bit warmer. I guess it would depend on the situation.