Derailleur compatibility

[Alex321]

But it's the shifter that determines the pull ratio isn't it?

Not by itself. It is a combination of shifter and mech.

The shifter determines how much cable is pulled for each gear shift.

The mech then moves a specific distance for that amount of cable (the pull ratio).

That distance must match the distance between sprockets. If you have shifters expecting one pull ratio, and mech expecting a different one (or if both of those are the same but the cassette spacing doesn't match), then you will not be changing cleanly one gear for each index position.

 

[Alex321]

Just to further note - the actual pull ratio is determined by the length of lever between the cable attachment point and the pivot point on the mech - which is why Raleighnut can vary the pull ratio by having two connection points.