Female cats will do that (mine have) and I can only guess it might be because they're not naturally as aggressive as males (though it's hard to tell sometimes) and do 'forget' what the point was halfway through. But put away trinkets or whatever for a while and DO get at least one other box (2-3 would be best) - it will make a big difference to future bathroom behaviour and if you look at this forum enough, you'll see it's a huge issue, one better sorted out in the beginning that later on.