The reason for this, if anyone is actually curious, is because the game counts “time” as “frames elapsed”. While the game is running at it’s ideal framerate (30 frames per second) the ingame time will be exactly right, but as the game slows down this will slow down the timers as well. The reason the game watches frames elapsed instead of watching the system clock or some similar more accurate timing mechanism is that the intention of the survival time awards is that you actually survive through that number of frames.
If, for example, the game watched the system clock rather than frames, slowdown would actually make the time awards easier, since if you were slowing down to 20 fps, you would actually spend 33% less time in game (and therefor only face 2/3 of the challenge) in order to gain the same award as someone running the game at full FPS as intended.
It’s possible that this could have been explained somewhere in the game to make it clearer how “time” was judges within the purview of the game, however I never really considered it to make enough difference to worry about. For some reason I failed to anticipate people pulling out stop-watches on me.
Anyway, sorry about the confusion.