I would have said there is no difference. It's a wee bit like one person saying after a race "John Scragbottom has broken the world record!" and another saying "John Scragbottom has just run the fastest time in history!" A lifetime best sounds more impressive than a personal best.
personal best is when the person is still alive and therefore can beat their personal best, lifetime best is when they're dead and therefore cannot beat their best. Thats my opinion anyway!
This is from the BBC website and is why I am confused:
"It was an encouraging start to the championships for Britain's heptathletes as Ennis and team-mate Sotherton both recorded lifetime bests in the opening 100m hurdles.
Ennis sliced 0.07 seconds off her personal best while Sotherton, 30, achieved her fastest-ever mark of 13.21. "
Maybe, as THECORBYLOON says, it's just different ways of saying the same thing.
They are the same but time frame is what makes the difference, once you achieve a new time, simultaneously its your "personal best" and your "lifetime best" because you have never done it before hence it's the only time it will be used interchangeably, however "lifetime best" is used after you would have set a "personal best" that you have not superseded over a period of time therefore whenever the term "lifetime best" is used you will never see "personal best" been used, you will see "season's best" because the athlete has a personal best in the past he or she has not replaced. Another way "lifetime best" is mentioned is after the athlete would have left the sport definitively for whatever reasons or dies.