When training in PA38s, my CFI used to regularly brim off the tanks & put 2 adult males in. I think in reality this is widespread. But aside from that, consider the actual weight for a typical cross country training flight:
People "in socks" plus:
Shoes, underwear, pants, shirt, jacket, pens, watch, cap, sunglasses, wallet per person
POH & aircraft documents
Flight bag with charts, prayer wheel, clip board, GPS, spare batteries, spare spectacles
First aid kit, strips, tie down kit, spare can of oil, towing bar & (sometimes) chocks
Sandwiches and drink bottle/thermos
Most of this clutter does not go into the weight & balance calc. I have always meant to assemble this mountain of stuff & weigh it all together - my guess is it would easily gobble up that 25lbs mentioned and some more.