Last year, when my train pulled into Derby station, the doors were released immediately, I was first off, and I checked the clock on the platform display as soon as I stepped off, and we were 31 minutes late arriving. However, checking RTT, that showed us as only 29 minutes late. Even allowing a minute to get the door open and step off the train, that is still a minutes' difference. I did consider submitting a claim, but as I had no proof of what time we actually arrived, I didn't bother.
Thinking about this since, on the platform displays at Derby, trains show as "arrived" long before the train actually gets into the platform, let alone stops or opens its doors. I think the train is shown as "arrived" when the train description steps into the platform berth in the signalling train describer. In the train describer, train descriptions step when a train passes a signal, so a train's description will step into the platform berth when the train passes the platform home signal (the signal reading into the platform). The train may still be a long way away from the platform when that happens, depending on how far away the signal is before the platform. This train description information is widely distributed around the railway, for use by platform displays, TRUST, etc. If my surmise is correct, trains are being recorded as arrived long before they have even got into the platform, let alone stopped.