I don't know the answer to that and it is frustrating. I suspect it has to do with the realities of setting a cut off date for deciding how much any post should be paid. Perhaps in the future it will become more sophisticated and able to cope with a continuum.
Meanwhile, I tend to recycle valuable "old" content in links etc in posts - not to raise the value of the original posts but to raise the value of the poster by introducing them to a wider network.
It's hard to place a value on the links in crude steem terms, but I think they may be important, as there is evidence elsewhere of the value of networks (and of having a good one). Happy 21 days!