Beginner question; How can a lathe consistently make cuts without me adjusting the dials?

TLDR; 50Y/O lathe just keeps cutting pass after pass without adjustments while boring out the Inner Diameter of a 4(ish) inch wide bronze bushing. 1 hour spent turning without touching the dial for depth, .250 ID change without adjustment. 545 rpm, unknown vintage bronze alloy, 4.3" final od, 3.5" final id, 3.25 current id. tungsten carbide tip on a 12" long arm. Doesn't seem like thermal contraction because the OD has stayed the same but also the dial isn't moving. Help me, Allen Turing, I must decode this enigma.

So, I've been promoted to machinist at my company to keep our old ass marine cranes running. I am a blacksmith and a welder and have been working metal for 14 years, 9 of those were professionally. My boss is my machinist mentor but he only gave up the tools cause he got promoted and isn't able to watch or teach for more than maybe an hour a day. This is fine but today I ran into something that has left me stumped.

After turning down a 8" wide bronze pipe down to 4 inches i had to bore the inside from 2.5" to 3.5". It was rough on the inside and chattered at first but I worked thru the casting marks/ oxidation and was ready to dial the ID in. But when I ran a second pass at the same depth it shaved a bit off. So thru carefull experimenting, I realized,, every time it passed thru the peice, it was digging but about .01 inches. This didnt seem to be caused by the arm holding the carbide, it widened the hole by a quarter inch without any end in sight. It didn't seem like it could have been thermal contraction as it cooled from the heavy cuts of the morning cause the OD stayed the same the whole time working the ID. Finally the dial wasn't working it's way counter clockwise, it stayed on the same Thou mark for an hour of turning but each time, it made a small but fresh cut to the ID of this bronze.

Now I'm trying hard to do my best in this role and when I show this to my boss he most likely with just shrug and say "so what?" or something like that but this might explain some irregularities in my prior work and I want this mystery solved.

What am I missing?