Is there an issue with your prints? The calibration tells your machine where the plate is. Some variability will be compensated for by the slicer. A span across the 300 mm plate of 0.6mm seems totally reasonable.
But I’m inexperienced. I read your post then some others here and came across a post where a forum user has a wider delta across his plate without issue. So I’d suggest you only worry about it if your prints are failing.