i got this bike off the street for real cheap and i'm taking it apart and trying to figure out what's up with it.

so i took the wheels off to put them on my other bike. the front wheel went on perfectly. the back wheel on the other hand...

i don't know how to speak parts, so i'll try to explain myself the best i can. the cogset is less wide than the rear fork. when i put the wheel on there is space between the fork and the cog. that is, if i try to tighten the nuts to install the wheelset, the fork tightens, like tensing a bow so to speak.

how can this be? isn't there a pretty standard width for a cogset to fit into a fork? these rims look pretty good and new (they look like deep v types) should i put some washers on the axle till the cogset is flush with the fork? is the cogset just badly built?

thanks