What do remote desktop tools do? TeamViewer allows mouse movement from even a phone touchscreen just fine, and unless they're computing the inverse of Windows's mouse acceleration, they must have another solution.
Some at least don't even bother to send relative mouse movements, what matters is where you click. So they disable the guest mouse-pointer and rely on the host instead and then only send positions every now and then and when you are actually doing something interactive.
Synergy types of applications don't have that freedom because the host mouse cursor don't extend to the other devices display.
On desktop clients, they fetch the cursor bitmap and renders it locally, then send absolute movements. For mobile clients, it is possible (and better) to send relative coordinates and let Windows accelerate that action, since touch events are not accelerated. Doing this acceleration actually makes the client feel more like a laptop touchpad if you are using relative mode.
Synergy types of applications don't have that freedom because the host mouse cursor don't extend to the other devices display.