While I'm very glad they're no longer in use in airports, I never saw a satisfactory answer to this. Perhaps I misunderstand the physics here, but wouldn't 10µrem across 100% of tissue be equivalent to, for instance, 1,000µrem across 1% of tissue?
Yes, that's correct. However, the difference in damage that that will do is still not clear. It's possible that skin tissue is more susceptible to damage than other kinds of tissue. (Or perhaps it's less susceptible.) Moreover, low-energy X-rays may do different kinds of damage than relatively high-energy X-rays or cosmic rays. Just looking at rem ignores a lot of variables about the long-term biological effects of the ionizing radiation.