I would also not forget the likely influence of available park hours on FP refreshing success at any given moment in time.
Let's pick Thursday 12/27/18 (just for kicks) this past year. MK was open 16 hours, Epcot 13.5 hours, DHS 12 hours, and DAK 14 hours.
Compare Thursday 5/17/18 (made up for comparison purposes), MK was open 13 hours, Epcot 12 hours, DHS 12.5 hours, and DAK 13 hours.
Or Thursday 2/7/19...MK was open 11 hours, Epcot 12 hours, DHS 11 hours, DAK 11 hours.
Every one of those hours adds or subtracts massive amounts of FPs to the system. Refreshing is a great tool every day of the year, you just have a lot more supply and a lot more guests in the system changing plans when hours are longer (and you can keep working it later in the day). I know this is obvious to some extent, just worth remembering the hours variability over certain times of the year.