Bosch came out with these mini-cordless tools using 10.8 Li-Ion batteries. I think Makita followed them into the market, at least in the US.
Then Milwaukee introduced their line of compact LI-Ion cordless tools and called them 12v. This is advertising nonsense as Li-Ion technology has a cell voltage potential of 3.6v per cell. You can make a 3.6v tool with one cell, or 7.2 with two, 10.8 with three, 14.4, 18, etc. You can't make 12 volts! If Milwaukee is reasonable in using no-load voltage then we are all driving cars with 13.2 volt batteries in them instead of 12 volts. Actually, Lead-Acid has a potential of 2.1v per cell so a 6 cell car battery is actually 12.6 volts.
If anyone cares, zinc-carbon or alkaline cells like a flashlight battery are 1.5volts per cell. NiCad and NiMh are both 1.2v so they actually could make a 12v tool by using 10 cells.
It seems like Bosch, etc. were forced to rename their 10.8 volt series, that each have 3 cells, to 12volts so that people wouldn't think that Milwaukee's 12volt series were better. I could see why certain users would choose Milwaukee since they have tools in the series that others don't have. I have some very old Milwaukee tools that are excellent but I don't feel that the new stuff is close to Bosch or Makita for quality.
Between business and personal, I have Bosch 10.8v and 36v Li-Ion Tools and Makita 18v Li-Ion tools. They are all excellent. The 10.8v tools get the most use. The 36v Hammer Drill with the slim pack battery is easier to use than the 24v NiCad Bosch and 1/2' corded drill it replaced. The 36v Rotary Hammer is so powerful and battery life is so good that I will sell my Bosch 11236VS SDS+ Rotary Hammer.