Trading observation

Just putting this out there for discussion.
Glancing through today's trades, and the major sell orders up the queue, I can't help but notice that the vast majority of the quantities are unusual numbers. When I buy or sell, I usually use a nice round number; it helps me keep track of my position and average and doesn't give me weird fractions of cents. I believe this would be the case with most people. So why are most of the sell orders for strange quantities? My interpretation is that someone is going to a lot of trouble to "paint the tape", this doesn't have the look of retail investors selling out. Just my opinion, I would be interested in other interpretations.
smsman