There is a maximum distance that the posts can push from the caliper body. It's possible that if the rotor is too thin and the pads are also worn, that you can can squish all the way to the bar and not stop. I've never had that happen even on worn rotors and pads but I generally maintain my stuff and it never gets too extreme.
Funny that you mention calipers and measurements of rotors. I have Magura, so spec is 2.0-1.8 and replace below that. A day or two ago I noticed the plastic digital caliper from harbor freight on my shelf so I thought I'd measure.
My brand new Magura rotors are around 1.9
My old Magura rotors with 2 hard seasons of mountain biking in lava dust are 1.7-1.8
My nearly new Hope rotors that never quite fit are 1.8
Maybe the caliper is 0.1 low. Maybe it's crappy enough not to use to measure tenths of a millimeter.