I have no legal knowledge; this is just what seems common-sense to me.
I don't think that the executor of a will has a prime duty to be fair between beneficiaries; their first concern must be to see that the wishes of the deceased are followed. If somebody leaves a house to one person and sums of cash to others, the Law must assume that this is what they wanted even if it turns out that the sum is zero.
If someone writing a will, leaving various assets, wants to ensure that beneficiaries receive set proportions of the estate, they should arrange their affairs to do that, probably by stating that all assets should be sold and giving those proportions.
It is because of problems such as this that we are advised to seek professional advice when making a will.