Originally posted by ychoed
Looks at lowest leveled skill that isn't maximum level.
If you had any 99 skills that weren't 99 yet it would go into that. If you have all 99 skills at 99, but have some of the 120 skills, not at 120, it then will look at the shown level and pick the lowest to put xp in.
If a skill is currently blocked for some reason it will ignore it. I.e you won't get archaeology xp, since archaeology is still under 6 month block.
It picks invention becuase your other 120 skills have a higher level. Xp means nothing for how it picks skills until you have max total level.
^This is spot on.