There is still a good amount of activity in here, so no need to fear not getting an answer. ME1 might be old, but plenty of peeps are still playing/ replaying it
However, if you look through the first page or two of this forum, you'll see that there are 2 or 3 topics already discussing this, so you may be able to find the answer you are looking for there.
Xbox keeps some hidden saves that can't be deleted manually by the usual means. People have posted a couple of workarounds for this. One thing you could do if you wanted is to reformat your Xbox HDD (putting the saves you want to keep on an external drive first of course) which will wipe all the ghost info on it, along with everything else - so make sure you backup all your saves that you want to keep before going this route!