How reliable are the grades?

I came across this video clip on Twitter - seems to be pretty important - someone is saying that grades are reliable to one grade either way - click in at about 12:19:48...
https://parliamentlive.tv/Event/Index/a3d523ca-09fc-49a5-84e3-d50c3a3bcbe3

Does that mean that anyone getting, say, a B, might deserve an A?

at the time yes, but this clip is 3 years old and refers to when grades were assigned via algorithm due to covid not letting students actually do the exams

I don't think so. Yes, the hearing was about the algorithm fiasco, but this question, right at the end, was looking forward to when 'real' exams return.

And I've found another video, much more recent, of an interview with the current Chief Regulator, Dr Jo Saxton. It's on YouTube https://www.youtube.com/watch?v=H5m-kMn8eHg. Click in at about 9:18, and you will hear Dr Saxton say something very similar:

"On that basis, more than one grade could well be a legitimate reflection of the student's performance".

Really?

More than one grade? So why is there only one grade on a certificate? What other "legitimate" grades might there be? And are any of those higher than the one on the certificate?