Normally poor quality bulbs, even bad contacts don't normally cause the problem, if you think about it that's what the flasher unit is doing...open, closed, open, closed.
Having different resistances normally just affects the flash rate, higher resistance faster flash and all that. I would buy a new set of bulbs from the same source just in case? To be honest I am clutching at straws for a reason why they are blowing though? All part of the joys of car electrics I am afraid ;-).