Just a quick search: the only study I found in my cursory search was one over the past twenty years or so, and it found that roughly 55% of dh were sweeps, 45% splits.
Need help from a mathematician here, but it seems to me, if you took any 2 games at random, you have 50% chance of winning the first, and 50% the second. So each team has a 25% chance of sweeping. I.e., the chances of a 'sweep' by either team are 50%. Looks to me that in the 'real world', there are more sweeps than would be expected mathematically; thus historically, it's a better bet that the dh will be a sweep than that it will be split. (This will be true regardless of a team's record--obviously for a team with a dismal record, the chances of a sweep by that team will be very low. For the RS tonight, somewhere around 20%).
It's quite possible I'm wrong here as it has been over a half-century since i flunked statistics.