I can explain
Your cartridge is obviously smaller than the chamber, has to be so it will feed in easily and smoothly and you can close the bolt without needing to put on too much force.
When the case is fired pressure is created pushing out on the brass. In low pressure cartridges the pressure is enough to expand the sides of the brass and pull the top down if you get what I mean. Like pinching 2 side of a balloon and pulling. The shoulder to base length becomes shortened while the width increases.
When you full length size the fired brass it pushes the sides in again and the top will push up again making the case taller.
Over your next few shoots measure the brass after shooting and after resizing. If you're full length sizing with the same loads you'll find the brass goes down when shooting and up after sizing.
Happens commonly in 5.56mm, low pressure loads for cast bullets etc.
It seems counter intuitive but there it is.
44 Rem. Magnum Marlin Model 1894
.280 Rem. Browning X-Bolt Hunter