I ran into this problem as well- I was a TA for 10 hours a week my last two spring semesters, so I just said Junior:5hrs/wk and Senior:5hrs/wk. The figures are pretty arbitrary due to recall factors and non-full year employment (like ours). I wouldn't sweat it too much... I imagine that adcoms are just trying to differentiate people who worked full-time, some-time, and no-time in order to put GPA/ECs into some context.