Understanding DHCP

Trying to understand how DHCP works

First case:

I had static IP assigned to two host machines (172.x.x.x), with one being the AP and the other station. Once the AP starts as a hotspot, it creates its virtual interface, and sometimes it assigns a dynamic IP (192.x.x.x) to it (overrides the static IP), and sometimes it wouldn't. Perhaps has more to do with how windows drivers are written (I suspect our drivers team may have modified the source code) but I'd been under the assumption that static IP can't get overriden.

Second case:
AP is running linux and a static IP is assigned to it (172.x.x.x). DHCP client is set on the STA, and I see a dynamic IP (172.x.x.x) is assigned to it. In a case where both machines were running windows, AP was assigned 192.x.x.x and when DHCP client was set on the STA, 192.x.x.x was assigned to it -- same subnet. Is it expected? How do both the IPs happen to be in the same subnet?