If the train in question was the one from Paddington due into Carmarthen at 21:20 on 14/11, then it was indeed recorded as 14 minutes late arriving into platform 2. There's nothing obvious that got directly in its way, but there was a potentially conflicting movement in the Pembroke Dock train that left platform 2 at 21:22, 12 late. That was well clear before 1B63 even got to Carmarthen Junction, from which it departed 12L at 21:29. But if it really does take 5 minutes to signal a new train into P2 once the station crossover has been cleared, that would be it.
Data further back than
RTT» are publicly available on Open Train Times, but for some reason actuals for that train are missing. A more complete version is however visible via
liverail. Rather clunky, but that does at least clarify that it (like RTT and
OTT▸ ) is just a viewer for whatever is to be found on the railway's own data feeds. Both of those sites tell you something about their data sources - in this case
TRUST▸ . However, you still need to know what those sources are drawing on within the railway system; for example TRUST data are tagged as "auto" or "manual" - I don't know what exactly that means, but doubt it's what you think.