Shimano RD M5120 and 10 / 11 speed compatibility

The cable pull ratios of Shimano "MTB" 10 and 11 speed systems (called dyna-sys10 and dyna-sys11) are different. The pull ratio of 10 speed is 1.2 and the pull ratio of 11 speed is 1.1. This change has been done probably in response to internal cable routing, which has quite large friction when compared to external cable routing and thus requires a pull ratio that moves the cable more.

How the 10 and 11 speed compatible derailleurs are implemented: the cable pull ratio is set to 1.15. Do note that on 11 speed systems, the biggest jump from middle cog (cog 6) is 5 cogs either way (to cog 1 or cog 11). So if the shifter expects a cable pull ratio of 1.1 but the derailleur has pull ratio of 1.15, it moves the chain sideways 1.15/1.1 * 5 = 5.2273 cogs. So there's 22.73% error at most, in gears you only rarely use. The limit screws tend to eliminate this error anyway.

If using 10 speed system, you adjust the cable tension such that shifting is perfect between cogs 5 and 6 (let's say cog 5.5) and then you at most use 4.5 cogs difference. Then the shifter expects pull ratio of 1.2 but sees 1.15, so error is 1.15/1.2 * 4.5 = 4.3132 or 0.1875 difference from the perfect shifting 4.5. Thus, you only have 18.75% error in 10 speed systems. In this case, the error is in a direction the limit stop screws can't eliminate.

Do note that due to various effects including friction, non-perfectness of cable housing, movement of cable end caps, etc. you can't even with a properly matched system have 0% error. The error of 22.73% is anyway eliminated with limit stop screws in 11 speed systems, and the error of 18.75% on 10 speed systems is the only that matters.

Probably Shimano has decided that the lower end groups have anyway so much inaccuracy that 18.75% error does not matter. After all, only an error of 50% or more results in a mis-shift. Also, according to http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/ the more accurate ratios are 3.9/3.6 = 1.0833 and 3.95/3.4 = 1.1618 so the difference is 1.1618-1.0833 = 0.0785 and the difference of 1.2-1.1 = 0.1 is just an approximation so the situation seems to be better than deduced based on 1.2 and 1.1.

You can't buy a combo 10/11 speed derailleur in the higher end groups at all.