When you run more than 5km, your Garmin running watch will look for the fastest stretch of 5km within that tracked activity, and use it as the basis for determining if you have a personal record.
You may not be confused, but incorrect generalisations like this are not helpful. Your statement is only true for SOME watches.I'm pretty sure I conceded that I wasn't previously aware that the statement didn't hold true for the Forerunner 310XT, thank you.
As the OP does not state which device he/she is using you may well be giving him/her false expectations.He/she did afterwards – and his/her device is not a Garmin running watch, so my earlier statement turned out to be irrelevant in his/her case anyway, and therefore there should be no false expectations arising from it.