Why does the definition for moderate drinking for men and women differ?

Men and women have completely different bodies, as women tend to be smaller while men tend to be bigger.
The smaller you are, the more lightweight you are. A woman being 115lbs and 5'3'', for example, is only able to consume so much, as opposed to a man who is 5'9'' and 185lbs. 
The effects of 5 drinks for that 5'9'' man wouldn't take as much effect, or wouldn't be as dangerous, if the 5'3'' had 5 drinks. 

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Basically create a schedule on what you need to study and stick with it