No problem, I understand (I was hoping... but I get it <grin>);
so, here is a jsfiddle of the problem:
Most of this is based on one fo the sortable examples from the main site.
So, after going to the example, the top sortable works as expected, but the bottom one (you may have to scroll down) does not. *(Make sure to have the window SMALL enough to require scrolling, it works as expected when you can see both).
I also put some basic instrumentation in to show my theory, based on what I could get from the jQueryUI code, it is comparing the clientX/Y to the item offset, so I am showing client X/Y and offset top/left in a box on the side.
You can see that the client x/y numbers are generally similar for both sortables, but the targetloc (from the ui.item.offset) is significantly different between the two.
Does this help?