well they will have some tests, experiments, calculations etc to determine at what point the risk of a food spreading disease and infection becomes unacceptable. i don't know how well the fda works, but i'm assuming it's a good organisation which does it's job judging by the replies so far.
so 3 rat hairs per 100 grams of chocolate for example might mean that there is potentially enough bacteria in 3 rat hairs for it to spread disease, but humans can tolerate the levels which would be on two or one.
i don't think the reasoning is like "three rat hairs? no way that's too gross", it will be more to do with science and numbers.
I don't think it works this way since almost all the food is pasteurized or otherwise disinfected before it's put into shipping containers. I think the idea is probably that if a factory passes these conditions, the food it produces is likely very safe to eat, and if it fails these conditions, it has demonstrated poor hygienic conditions which means it may produce the occasional shipment that gets people sick.
for the alarmists, you're probably going to find tons more human skin flakes or hair or spittle and such or even industrial grease and oil than rat shit in a factory manned by workers, you know. it is still tons more clean than catching and eating food yourself (can you imagine what kind of shit goes into fresh-caught fish from a river or a recently slaughtered cow?)!
these statistics don't bother me either; the US has some of the cleanest food in the world (come on, can you imagine what the standards are in BELARUS or MONGOLIA or even CHINA) and besides, you probably get nearly as much filth as is regulated simply by eating food on a daily basis through silverware, your hands, and breathing in dust.