I suspect that maybe there is a difference in the thickness of the envelope. Like was mentioned in a thread a while back, there may be a template that would allow a FLAT envelope of a certain size to fit through. These envelopes are probably designed to take a stack of papers or books that are up to a certain dimension, 11 x 14(?) or 9x11(?) by a certain thickness, say 2 or 3 inches. That can stack flat in a tray or cart. You can put larger items in them by letting excess length or width rise over a larger thicker item in the center. That might change the classification of the parcel to a package rather than an envelope since it would exceed a specific dimension (thickness). In other words, if you take larger items and fit them in they might not be flat. That would be my suspicion. I could be wrong though.