At St Ives, our food plans for the return were based on 85% of riders passing through - 80% finishers and 5% out of time riders - but we were happy to feed those who were making their way back out of time (within reason). These statistics (as I recall) would have worked well in 2009 (wet and windy) and 2013 (hot).
On the return leg we kept waiting for a bulge of riders that never came. Reflecting on the times at which riders came in, riders of all speeds and capabilities were later than expected (despite Phil D having been a controller in 2009 and 2013 and myself having ridden in 2009 and catered in 2013). Even the strong riders looked fairly shattered after crossing the fens in the headwinds.
Putting these factors together I would suggest that the weather was the main factor, and discussions about a relaxed first-half increased the impact of the weather. I'm sure that this will serve as warning for riders in 2021 - both through discussion groups and (dare I suggest) guidance in official briefings - and riders will give themselves more time for a potentially difficult return.
That, plus the early entry that should ensure a higher proportion of more experienced riders, whilst retaining the principle of not requiring qualifying rides, should, IMHO, help a return to a higher level of completion in 2021.
