Why do you think that he was not a top bowler. He was ranked 1-2 for 1-2 years in his career. He was mostly ranked among the top 5-6 in ODI bowling during his entire career.
Anyway, Top all rounder doesn't have to be top batsman or top bowler. It's about bringing enough in batting, bowling and fielding.
About Afridi - Afridi was ranked among the top 20 batsmen for zero days. Kapil was among the top 20 rank in batting for a entire decade. Clearly, 23 average in 80s is not the same as doing it in current era.
If you still want to see how they stack up in
all round performance,
View attachment 76932
It's clear that Kapil held top rank in all round category in pretty much entire 80s. He did it due to being among the top 20 batsmen in entire 80 and among the top 5-6 bowler at the same time. His fielding was top class as well and that's not even accounted in ICC ranking.
It's fine to put some other names, but Afridi's example due to batting average is a wrong one here.