I may be mistaken, but you seem to be using the same logic one would use to assess automobile performance. With a car, power to weight affects one of its major performance characteristics: acceleration.
With a plane, power to weight still affects acceleration performance and (ultimately) top speed. But it has less to do with how high the plane can fly. Altitude is determined by the design of your wing and at what altitudes the engines can function.
Remember, as you go higher there is less air. Less air means that the wing produces less lift, and it also means that the engines will often produce less power. You can compensate for this by other design means (like adding a turbo to the engine for example), but if you don't eventually you'll get to an altitude that you can't get past.
To be fair, both of these aircraft are designed for low altitude flying (ground attacks, mostly). So it would be mostly pointless to fit a bunch of stuff to allow the craft to fly much higher than a couple miles or so. Thus I'm willing to bet the engineers just skipped out on that stuff to save weight for other things (like guns, missiles, armor, etc).
This, of course, all assumes that the information being given out on the craft is accurate. I'm sure it's all top secret, and I doubt Russia or the US are going to just hand out hard numbers.
jwenting commented that, in reality, the difference between high altitude and low altitude design generally has to do with optimization, not adding "stuff". This is a great point, for some reason my head was thinking "turbo/supercharger" (which doesn't even make sense with jets). I would maintain that, in many cases, you are adding stuff to get to a higher altitude (pressurization stuff, for example).
But, and this is jwentwings main point I think, when you look at the aerodynamics of an aircraft the main difference is a matter of optimization. A wing that has good lift, good maneuverability and good stability at low altitude/speed, is not going to be so great at high altitude/speed.