11-pitch is a very common size; 10- and 12- might be more familiar sizes in North America, but when you move to machines made in Europe 11-pitch is widely used. Olympia listed its most popular type options as being 10-, 11-, 12, and 17-pitch typefaces.

Fleetwing wrote:

I wonder whether Olympia made it physically impossible to mount the wrong pitch carriage on a typewriter?

The carriages are mechanically different from one another. For example, you can physically mount and lock a carriage from a pica machine onto a different machine using elite, but the carriage release will be permanently engaged and you won't be able to type with it.

I've had a chance to take a quick look at a few machines, and I would assume that matching frame and rail numbers should be the norm rather than the exception. I'd also speculate that when the numbers match that you have an original carriage and body combination, and if they don't, that it's a carriage that was part of another machine and someone got them mixed up, as you could understand might happen in a large office using a number of SG1s. 

I also noted that the rails on machines made between '61 to '63 don't have numbers at all. This leads me to guess that either Olympia stopped stamping the serial numbers on rails during the last few years of SG1 production, OR, they happen to be carriages that were bought on their own and it would make sense that they wouldn't have a serial number associated with a specific frame.