it is the shells- I have had a UTG, and now use a DE; and we have several UTG, DE TM and Crosman shottys in use at our field and
all of them do it; in fact getting 4-8 BBs is so common with new UTG shells we disparagingly call it a 'UTG special' as though you get more BBs they only fly half as far which is usually no where near where you were aiming
Most of my new shells stopped doing it after I left them fully loaded for a week but it still happens off and on. If you look at the breakdown for these 3 BB shottys you can see the mechanism that is supposed to 'prevent' more than 3 BBs from loading but nearly every shotty I have seen has done it at least once and if you search enough forums you can see that even TM shells can have the problem (and that currently there is no fixed solution as everyone seems to need to do something different to fix the issue)