Well Paul I'm sure you know that to have as many cameras in that hall as there appeared to be would cost about three times the budget. So there were not in fact that many.
I wasn't there, so I don't know what happened, but the normal approach is to film each song two or three times. The audio is then taken from the take in which the close-ups are recorded, and the wide-shots are post-synched to that audio. When the act is miming to a pre-recorded track, this usually works seamlessly, but when it's done two two or more live performances it doesn't always fit perfectly.
I spotted a couple of very minor sync issues, but they were well within normal tolerances and I certainly wouldn't mark the show down for them.
The alternative is either only to use the available cameras on a single take and look 'shot-poor' in comparison with other TV music shows, or to ask for triple the budget and have the show idea turned down.
"Becky Unthank's solo was inaudible whilst her sister's harmonies were as clear as day" - not on my telly, just not perfectly balanced perhaps, but then a few things usually do slip through the net and we hope people won't mind too much.