Hoody, this is very normal. I personally believe it's because sitting is a very un-natural position to poo in. Having lived in Asia for a while, I discovered that sqatting is actually a much more comfortable, natural position. I think this is why it takes kids so long to be comfortable with pooing on the toilet or potty.

I am not really sure what the answer is. I am still struggling a bit with this with my 3yo - he mostly will use the toilet, but goes through phases of pooing in his pants for days at a time. The thing that has worked best for us, is to make no fuss at all when he dirties his undies, but to praise him like crazy when he does a poo on the toilet. We also use a sticker chart - each day he keeps his undies clean he gets a sticker and five stickers gets him a new toy (like a matchbox car or similar). We have had a lot of success with this actually but he is older than your DD. Could be worth a try though?

FWIW, I don't think this reluctance to poo on the toilet is likely to lead to any longer term problems, it is just a matter of them finally getting used to it.