Ultegra 6600 right shifter lever alignment
The misalignment may be caused by damage to inside of one of the shifter mechanism components you circled on the photo. They all stack up on a center post, and some have interlocking components to align everything just right.
If one of these components is damaged (i.e. stripped out, chipped, etc.) it would not stop at the correct position, and keep rotating past the intended &quot;stop&quot; position. This is the most likely scenario, since the area you circled is not part of the 10spd detents; those are the smaller teeth closer to the front of the shifter body. If it is a little floppy when you first push it (no cable attached), this may be the case.
There may also be a damaged bit of plastic that stops the lever in the correct position. I'm not 100% on the 6600 levers, but the mechanism is very different from what is in 6700+ levers that are currently out on the market. This was the last generation of this design before Shimano redesigned it completely.
Echoing the advice above, I would not recommend trying disassembly/repair, unless you already have  a backup shifter ready to use. Most shops, as a rule, will not attempt to repair this either. If something is broken, it will probably fail  completely at some point in the future.
WD-40 is definitely not great for the shifter. The grease Shimano uses in them hardens up if they sit for too long; you basically have to disassemble these shifters and scrape off the old grease with a pick or flathead screwdriver. WD-40 won't dissolve the grease; it's a hydroscopic solvent that will absorb water and anything dissolved in it, and eventually gum up again. Plus, it gets everywhere!