I think you will find it is all about the odds game. At dispatch, if you only have one pack, you plan at a lower level because you are departing under a MEL with reduced redundancy. You may have to load extra fuel. If you are already up there at FL 370 with two packs, and one fails, you can carry on at normal cruise levels. The reasoning is probably because a) the odds of a second pack failure on the same flight is remote and b) you may not want to compromise your fuel reserves by descending to FL250.
The way I rationalise this thinking is that we are not required in certification to assume that simultaneous with an engine failure on takeoff the gear won't retract, or the other engine will fail a few minutes later. However, if you are doing a gear-down ferry, you do have to consider one other failure (an engine) in your performance calculations, because you are planning a flight at a reduced level of redundancy.