lots of time hamsters just very tentatively bite - to see your reaction - to see if you're edible lol - to like see what you are. its one of their ways to figure something out, like what you are, why you're there etc. as you probably know their vision isn't great so they can't see you well, if at all. they can smell you but that just makes them even more inquisitive
at first my robos dashed around my hand, every so often biting my finger very very gently and tentatively
its not great - i knkow but sometimes they are just tooooo curious lol
and if you stick your fingers through the bars thats either them thinking "oooohhh food!!" if you usually feed them treats through the bars, or thats them being territorial and not wanting anything to invade their space. usually.
they also dont like yoru hand to smell of cream, soap, sprays, well they do like it to smell of food thats sometimes y they bite XD. and like anything smelly. which is why i alwayswash my hands b4 touching them - especially if i'm with the robos lol.
goodluck!