Haha, he's not even trying. Carl Tausig has three Opus 1s, and he didn't compose anywhere near as much as Clementi (every so often, he'd have a change of heart and renounce all his earlier works).
The complete edition of Clementi Sonatas and Sonatinas I have (Braunschweig) also has two Op36s which line up with your editions. I know some Clementi Sonatas were later rebadged as Sonatinas, though that wouldn't explain what happened in this case.
Opus numbers seem to be quite a bit screwy for a lot of composers at this time. Maybe when the new complete critical edition of his works is completed (now underway) they'll sort it all out (and I guess we'll have another letter to show off with, a la BWV, K, D, S etc..)