The books never "NEED" that many games in a single day. The majority of the time they will just need one side or total. Like a few days ago they need the Nationals in GM3 of the series with the red Sox. Today, all of the squares were on the over in the Toronto game, which pushed or won for the books depending on the line.
I'm not sure where you are getting the percentages, etc. but the sites that I know that show that kind of stuff show percentages - not amount bet. It matters not if 75% is on one side if all the big bettors are on the other side. If up to date info of just how much a book has on a particular side would ever be made available publicly, it would be a gold mine. But, since that will never happen...LOL.
Good luck..