I agree this is a puzzle. The date in the first line of the will (if I'm reading it correctly) seems to be
M ccccc L the xviijth day of ffebruarie.
I take this to be 18 February 1550, which would be 1551 in the modern calendar. But I think this is not the original will but a later copy, because it isn’t signed by the testator or the witnesses. All the names at the end are in the same hand.
The date of the inventory, apparently 9 August 1549, and of the administration, granted 18 March 1549/50, are consistent with each other, but not with the will, even allowing for the year beginning on 25 March. Yet the contents do suggest that all 3 documents relate to the same person.
All I can suggest is that the date given in the first line of the will is the date this copy was made, and not the date the original will was written. But I can’t say I have ever seen this before.
Hampshire Archives have catalogued the will as 1550, as you would expect, since administration was granted in that year. But you might run it past them and ask their opinion about the date of the will itself? I see there are no parish registers for Wellow before 1570.