I find that to be mostly true with my handmade soaps too marinesoaper. I think thats common. For me, it seems like all soaps leave some residue but the natural soaps seem to leave a much smaller more natural amount of residue and is not hard to clean up. I can't say for sure but I believe its because the handmade soaps are not full of sulfates and other chemical 'detergent' type additives. It seems to me if there is a harsh scum sticking to your tub its probably sticking to your body as well.
I know that there are some more knowlegeable members on this forum that may set me straight if I'm talking crazy talk.
lol...but that's what I'm thinking. I wanted to post because its an interesting topic.