User ID tracked in prop shows more visit than evar why?

AA is reporting correctly.

Your prop tracks the value when that value has been set in a hit.

Your eVar also tracks the value when that value has been set in a hit. But since you have configured to have "Never" expiry, then that value gets attributed with all subsequent hits forever until a new value is tracked.

Here's a simple example:

Hit 17: prop set with "123", eVar set with "123".

Hit 18: prop not set with anything, eVar's "123" continues to be attributed here.

Hit 19: prop not set with anything, eVar's "123" continues to be attributed here.

Hit 20: prop set with "789", eVar set with "789".

See how the value "123" has been set with 1 hit based on the prop, but 3 hits based on the eVar.

So when you have your segment where the prop does not exist, the segment includes hits like Hit 18 and Hit 19. Yet, since the eVar value persisted, so when you have your segment where the eVar does not exist, the segment does not include Hit 18 nor Hit 19.

And so you can expect fewer visits with the eVar-based segment than with the prop-based segment.