What counts as a PB?

I've seen a lot of people saying "my race time was this but my watch says this" in terms of a PB in a race. Some people say a PB is how fast your GPS has ever recorded that distance, some say it's what the official race time says. I'll be honest this is a subject that I have massively changed my opinion on in the last year or so. I recently ran a 10K PB at Silverstone. Watch said 10K was in 48 mins, official time from race was 48:35. I was all for the theory that your PB is what your watch says your PB is. However, I no longer subscribe to that way of thinking and here's why:

1. Courses are measured. Sounds simple but courses are measured accurately and watches have a slight deviation. It is frustrating when you run further than the distance of the race but I've also had some where my watch says it's just short so it goes both ways. A course should be officially measured and based on that, your PB should be taken as your official race time.
   
2. 
   Watches/phones can be wrong. Shock horror! But yes they can be wrong as you can see below. My mile record is world record level. So I can believe my watch or I can use common sense! A course doesn't deviate, have mini drop outs or get your starting position slightly wrong.
   
   
   
   
   
   
   
   
   
   
3. It's what Mo would say! I'd imagine anyway. I can't think if you asked Mo Farah what his 10K PB would be he would say "well I did X time in training once". It would be his racing PB.
   
4. It's a level playing field. Chip timing or officially timed races are the only accurate way to measure one athlete against another. Even within watches there's variation. I do lots of long runs with my brother who has a different Garmin to me and despite running side by side my watch always seems to be slightly behind distance wise. Chip timing and officially timed races take away all those variables.

It's frustrating as most of my "records" in races are quicker on my watch than my official race time but all things considered, the only open and fully honest way in my opinion to say what your PB is is to go by what your official race time is.