That makes absolutely no sense
Ok, think of it like this.
When a key is entered, if it has no name registered, it asks the master server what key number it is. For example, when Badspot put his key in, the master server said his was the first key, and dubbed it Blockhead1 for ID 0. That explains the old offset.
Now, when the key for ID 9999 was entered, it was the 10,000th key, and was given the name Blockhead10000. There hasn't been a key for ID 10000 yet, thus making the next ID 10001. Since ID 10001 is now the next ID after 9999 (the 10,000th ID), it is the 10,001st ID, giving it the name Blockhead10001.
Er, following me? >_>