I don't understand why AWK doesn't compare properly after using the sub() function.

When I run:

echo -e "LG:1000\nLG:3" | awk '{aa=$1; sub("LG:","",aa); if(aa <= 20) {print $0}}'

the output is LG:1000 instead should be LG:3.

If I run this command:

echo -e "1000\n3" | awk '{aa=$1; sub("LG:","",aa); if(aa <= 20) {print $0}}

It works properly the output is 3.