Non-pro rata means that beneficiaries don't take equally. The provisions you have provided don't provide enough information to confirm what the testator intends. It may be that the will designates specific devises in other provisions and it was not the testator's intent to divide the estate equally. For example, if he gave one child a 2 acre property, another child a condo, and another child his favorite watch, the will determines what each beneficiary gets but the inheritances are non prorata.
If this is already in the probate process, please consult with the estate attorney to advise you.
In general, this provision is to allow the Personal Representative discretion to distribute the assets of the estate in some other manner than proportional shares of each asset. In other words, if there are six children, it is not necessary to give each child one-sixth of each asset - they can receive non-pro rata shares, so long as the ultimate value of the total shares is equal.