James Fidell writes

No idea why some places aren't free all the time when others are,
though. Seems like an odd policy. Perhaps they don't feel they gain
enough from being an RHS garden to make it worthwhile. Or perhaps it's
an attempt to manage visitor numbers?

Most of the gardens are not RHS-owned gardens, are they? So they are
simply taking a commercial decision to attract a few extra people, free,
when numbers are low, but not foregoing income when visitor numbers are
high. Just like 'Orange Wednesday' - two cinema tickets for the price of
one on a day when hardly anyone wants to go to the cinema.
--
Kay