It seems to go 1 over whenever it has to create new keys. I have noticed the same several times, but I dont know why. My assumption is that it expects a change address to be used with the next transaction and thus creates one more.
I'm pretty sure it is just bad counting. I think it fills the keypool until the specified index (number you entered) has a key. The problem is that it starts counting from 0, so when you get to say index 10000, the size is actually 10001.