Why are girls usually stronger in their lower body lifts compared to the upper body lifts, when compared to men?
Are you comparing novices or experienced lifters? If you take two untrained & unathletic individuals, a man and a woman, they will both be weak, but the man will probably be stronger. However both may have a baseline of strength and conditioning in the lower body from walking, running, climbing up stairs etc... that their upper bodies lack. The effect of this would be more pronounced in a woman who has a lesser natural adaptation for strength than a man of similar background.
When it comes to trained individuals, women can display a higher endurance level when working with near maximal weights than men because their central nervous system does not adapt for strength to the extent that a man's can. A bit of guesswork: as larger muscles have a greater capacity for strength even in the absence of significant neurological adaptation, the lower body will be even stronger proportionally in women. The same applies to weak men (assuming they're training correctly), though usually to a lesser degree. As strength increases to elite levels, the conventional ratios expected for lifts kind of fall by the wayside. For example, top level powerlifters have squats much closer to their deadlift (sometimes equal or greater) than the average trainee.
As Sean said above, biomechanics probably play a role also.