Litterbox issues

So i've written about this issue before, I have two kittens both of who use the same litterbox, one (the younger one Malcolm) always uses the litterbox fine, digs, pees/goes number 2 and covers like a good kitten.

The second kitten (Bailey older by one month), goes pee in the litterbox fine, (ie. right in the middle or 'near' the middle however you want to look at it), but when it comes to going "number two" she INSISTS going on the edge of the litterbox IF AT ALL, its like she's in the litterbox but she is 'going number 2' outside of it... like its on the edge almost touching the gorund "number 2" and then we have to wipe it up, etc.. etc..

We have no changed from the 'standard' litterbox i.e one you pick up at Wal-Mart/Petsmart/Petcetera/JUST YOUR NORMAL LITTERBOX SIZE, to a top of the rubbermaid container that is a bit wider and longer, and... well she still went outside the litterbox.....

I'm wondering if anyone can shed some light on this...? Because I am baffled, I would say that maybe JUST MAYBE it was because of the type of litter we use (we use wood pellets) but then I realize that she has no problem going pee in the litterbox in the middle of it, so I'm just as I said earlier 5 lines up BAFFLED.. ha ha anyways any ideas from anyone that would be great!

Thanks

Well, I can't tell you what might be causing it but I can tell you we had the same problem with Elliott.
He was 18 months old when we brought him in, he'd been stray all of his life.
He's two now.

He was using an uncovered litter box and he'd pee in the middle , but he always pooped right at the edge, he'd stand inside and hang his butt out over and sometimes the poop went in the box, but over half the time it was outside the box.
First I tried putting plastic under the edge of the box so when he did "miss" the poop wouldn't go on the floor.
That worked for a while until he decided that the plastic was what he should use to cover up his poop...what a mess.

I bought a covered litter box and he's hasn't hung his fat little butt out over the edge since.

it could be the litter type. have you tried a 2nd litter box with regular litter in it to see?