Something to do with sunspot activity and the higher doses received in higher latitudes.
A fact little recognized in statistics (because it is inherently non-statistical) is that radiation doses from extra-terrestrial sources (the main factor in altitude flying) can vary from zip to phenomenal at any time and place. The natural rad sources in space - whether exploding galaxies or just radiation hot-spots on the sun that spin around to point at earth for days, hours, or seconds - can vary over an energy range of 1,000,000x or more.
High ratios of variation apply to surface radiation levels as well (a point to ponder in relation to 'global warming') but the proportional differences are especially notable high up where atmospheric filtering catches less of the incoming energy.
So it's another kind of lottery. Dosimeters might not be a bad idea for aircrew, but their use also might rock the employment boat more than one would want.