Just a general guideline
Clearly, the 50 recommendation is somewhat arbitrary. If I had the choice of sitting home doing nothing, or playing a game with a 45 SCORE, I suppose I'd rather be playing than contemplating my navel!
Also, as pointed out, a game with a 40 SCORE, but with two players at the table, instead of the four assumed for the sims, turns out to be superior to the 50 SCORE.
So, take the 50 with somewhat of a grain of salt, and don't consider it as a hard-and-fast "rule," which it was never intended to be.