It is possible that following the death the family thought there was no need to prove the will (possibly because the estate was of modest value and there was agreement in the family on how to handle the estate) but decades later some legal issue came up (such as sale of a property) which compelled them to prove the will.
I've seen a number of cases in the UK where land and property has been informally inherited over a number of generations and it has only been when a sale has had to be made that up to a century's worth of undocumented and informal transfers have had to be regularised.