Why the number moves
The thresholds in the Act turn on how many individuals may reasonably be expected to be present at the same time. That is not the seated capacity on the fire certificate, and it is not the figure on the licence. It counts staff, contractors and anyone else on the premises for the relevant activity, and it is assessed against the largest realistic occasion rather than the typical one.
Most organisations start from the number they already have, because it is the number they already have. When the calculation is done from first principles, premises that were confidently standard tier turn out to sit on or over the enhanced threshold, and the duties that follow are materially different.
The correction is not complicated. It needs the right basis, a defensible method, and a record of who did it and when. Do it before the tier drives every other decision you make.