weird.  well, do you think the axe works if you inched backwards up his head a little more?

There are 2 positions you can stand on at the lowest point of the worm, actually the farthest position never hits, but the one closer to the worm you're standing on usually will, it only won't if the worm hasn't moved since teleporting. A lot of weapons are affected by this in some way.

And by 'moved since teleporting' I mean things like jumping, getting damaged, etc. Just walking doesn't change that state unless you walk off a cliff or something. It's because your y-value is something like 1 pixel above land after you tele, and after you start jumping around it will be somewhat random between 0 and 1.