why do women get paid less than men?

From the numbers I have seen women do not get paid less than men. Women make employment choices that are different than men. Women take more personal days for whatever reason. I saw an interesting sight one day. There were some spools of wire that needed to be moved upstairs. Two young healthy men put a rod through the spool and each taking a side walked up the stairs. Along came 4 young healthy women. They too put a rod through the spool but they lacked the upper body strength so the four of them moved the same spool up the stairs. So which is more fair to pay the girls 1/2 of what the men did since they did 1/2 the work or pay them both the same and punish the guys for being efficient?