The first part of your question is a strange oddity I don't really have an answer for. May have something to do with the book trying to balance their money on the two seperate markets independently of one another. Just take the better odds, as you say.
As for the totals question, hopefully the following example will help you out. Using your 'Over 2½,3 +113' example, let's say you bet it for 50 bucks. 25 actually goes on at +113 o2.5 and the other 25 goes on at +113 o3. Result ends up 2-1, you win the first half (o2.5) and push the second (o3).