Syncing through USB always has some jitter, as timings of when the Clock messages arrive are always a bit inconsistent, as the USB Host keeps scanning every connected USB device for new input. This is how USB is designed. These fluctuations shouldn’t matter much in practice though, devices should still be playing the same beat, as this is dictated by the number of actual Clock messages received (a quarter note is exactly 24 clock messages), not by their timings. The value that Ableton displays is the current BPM rate estimate, based on the jittery Clock messages it receives. It should still keep overall sync to the same beat with the rest of the devices.