When the snap-to targets are in a different offset parent from the element being resized, their coordinates appear to be incorrectly calculated. Using the same snapping options with $.ui.draggable works. I think the difference might boil down to this:

$.ui.draggable.snap uses $(this).offset() to get each snap-to targets' coordinates at https://github.com/jquery/jquery-ui/blob/master/ui/widgets/draggable.js#L998

$ui.resizable.snap uses $(this).position() to get similar coordinates at https://github.com/polomoshnov/jQuery-UI-Resizable-Snap-extension/blob/master/jquery.ui.resizable.snap.ext.js#L24

I'm going to try to see if I can get it fixed and I'll submit a PR if I do. Regardless, I'll try to do up a plunkr/pen/fiddle/bin/etc to demonstrate.