I just solved it myself:
Because of the animation that happens on the removal of a repeater row, there is a 250ms delay before the values are removed.
by using .on('remove', function())
from jquery-ui the problem is gone.
here is my working code:
$j("." + currenttab + i).on("remove", function () {
calculate_subtotal(currenttab);
})
Thank you for your help!