Actually, I realized after I made this post that the number of series the Cubs has is because of the 16 teams in the NL versus 14 teams in the AL.
Though this does beg the question of how the NL teams are selected that play fewer interleague series. Why the Cubs, for example?
I'm just guessing it's because there are
six teams in the
NL-Central. I didn't look at the schedule to see if all the teams in that division do that. They do have an extra team in their own division to play. The only division in MLB that does not have five teams in a division is the
AL-West, which only has four. It seems like they could achieve balance by moving an NL-Central team to the AL-West, maybe Houston. (The NFL did this sort of thing by paying a large fee to a team willing to move, most recently being the Seattle Seahawks moving from the AFC to the NFC.)