I'm not an expert (just looked at some old course notes) but bear consider the following:
Some airlines use larger crews than the legal minimum either due to industrial agreements or to improve cabin service. Some US carriers pay (or paid) an allowance for working with a smaller (though still legal) crew than agreed with the union contract.
The FAA require additional crew for long sectors.
Some airlines will on occasions leave a cabin empty to allow operation with a smaller crew.
As already mentioned there may be additional training crew and provision is needed for them.
A member of crew may be on the flight deck for landing - but only if any jump seats are not occupied.
Hence the number of cabin crew sat in jump seats will fluctuate from day to day.
Last edited by Peter47; 28th November 2016 at 16:54.