Inconsistent seating depth

Loaded approx 20 rounds with a certain seating depth set and made no adjustments throughout the load. After 20 rounds, I measured the base to ogive and at least 5 of them at more than .001 difference, mostly negative. What could be the reasons for it? Using a lee turret as a single stage on a desk that isn't fixed to the ground, slightly slides if applied enough force on the lever of press. I did measure the case before reloading and all were + or - 0.0005. Could be the movement of the table, but I don't understand why, as the micrometer seating die is set, and no matter the movement, it cannot push the projectile further in? Just trying to understand.

Thank you