Two things.

Firstly, most ESCs have a "neutral zone" that's designed to eliminate jitter around zero throttle, which might explain the "nothing happens until 1100" observation.

The second is that it's most likely the way the ESC is mapping the inputs to outputs.

In both cases, it's coarser than what I'd expect. It's pretty common to have a neutral zone of around 5% - yours looks to be 10%.

Unless there's something unexpected going on in how the arduino code is mapping the intended PWM value to actual signal, it's going to be entirely down to how the ESC works. Do you have a 'scope or similar to inspect the actual PWM signal from the Arduino?