A PLC doesn't watch its inputs continuously — it takes one snapshot, runs the whole program against that frozen picture, then reports the result. Repeat.
It's tempting to picture a running PLC as something that continuously watches its wired-in field devices and continuously drives its outputs, reacting instantly and constantly to whatever is happening in the real world at that exact instant. That mental model is wrong, and the actual behavior matters a great deal for anyone troubleshooting a system or wiring up a fast or brief signal. A PLC executes a strict, repeating three-phase cycle called the scan cycle: it reads every physical input into memory once, at the start; it runs the entire logic program against that frozen snapshot, top to bottom; then it writes every physical output once, at the end. Only then does it start the next scan and read inputs again. Nothing about a physical input is visible to the logic mid-scan — the logic only ever sees what was captured at the last read.