calculation of PB

HI 

I came across this and the more I dug into it the more it puzzled me.

I went for a 10k run and i beat my PB - i got under the 50 min mark for the first time - Yay!

I ran 10.01km the last 10m is me fumbling for the stop button on the watch

The total time for the activity is reported as 49:58

When I go to the laps and check the Cumulative Time for the first 10k I get 49.53

When I go to the list of Personal Records I get 49.58

So for some reason the last 10 m and the accompanying 4.5 sec are counted in to the 10k record.

That is strange - the 10k time should be 49.53

More so, when I look at my PB on 5k. That was set on a 7k run - but it was calculated as the fasted 5k continuous stretch of the 7k

I know I am counting seconds here - but that's what runners do i guess