Shimano tried to use the same rear derailleur ratio (SIS), but then came 9-10 speed. They had to decrease the cable pull each time. This got pretty bad on 10spd. 10 speed SIS road systems are very sensitive to setup, or dirty, old cables. They pull very very little cable so everything needs to be very precise.

Shimano then introduced 10 speed on mountain bikes and used an entirely different cable pull and derailleur ratio so the system works overall better, especially in harsh environments. Dyna-Sys. A Dyna-Sys 10 speed mountain shifter pulls more cable than a 6 speed shifter...

When 11 speed came out Shimano changed the derailleur ratio on road stuff too, so the shifters can pull more cable. It works pretty good, but this means no backwards compatibility for SIS rear derailleurs. Later this trickled down to Tiagra 4700 and now Sora R3000. (SRAM with their 1:1 and I think Campagnolo have been doing this for a long time.)

What I don't understand is that why the cable pull and derailleur ratio is not the same on 11spd road and MTB stuff... I know that more cable pull is better for MTB, but I don't see a downside if road stuff used it too. You could mix RDs.

I've heard that people bought Sora R3000 rear derailleurs for older 8-9-10 speed stuff and it did not work.

I know that Tiagra 4700 uses the same derailleur ratio as 11 speed road stuff. Maybe this is the case with R3000 too, I don't know, I only know that it is definitely different than SIS.

A good article about all of this with some useful data (sadly nothing about 4700 or R3000 yet)

http://blog.artscyclery.com/science-...compatibility/