Another way
I did some quick numbers on Notepad and came up with these odds (for single-deck):
Under 13..... 44.947%
Exactly 13..... 8.446%
Over 13..... 46.606%
Emil didn't specify the game he was playing, but in a single-deck game, removing just two 3s from play gives an off-the-top advantage on the over-13 wager. The numbers, assuming 2450 (50 * 49) possible non-order-specific combinations:
Under 13..... 43.020%
Exactly 13..... 6.531%
Over 13 ..... 50.449%
Obviously, it doesn't require an illegal act to ensure that two 3s are kept out of play. A third 3, or an additional A or 2, would increase your odds on the Over-13 wager even more. Ideally, you'd prefer to cut a 2 out of play, leaving the Aces for your blackjacks.
I encourage anyone to confirm these numbers before accepting them as gospel. :)