Unfortunately, you've discovered the limitations of a drum sander. I have a Supermax 19-38 and I've had similar results. It is basically snipe. I think it is related to two things. One, when you first feed in the box, the drum is spinning at full speed and first hits a corner which progressively becomes wider as it goes through the sander. The first and last corner to go through gets a few more revolutions of sanding and get thinner. Two, when the first corner goes in, the hold down bar/roller presses it down but not as far down as both rollers which together push twice as hard. I've found that I can push work pieces down even more with moderate finger pressure as they enter the sander and make this a little easier.

I'd stick down a sheet of 120 or 150 and have those edges in the same plane in just a few minutes. I save the drum sander for those pieces that won't have parallel edges nearby to judge them by.

Dan