A while ago, I looked through the desk in my home/office and collected all of the stamps that I had laying around.
It turned out that I had two partial rolls of stamps, neither of which contained stamps equal in value to those I needed to mail envelops. I went to the post office to get some small denominated stamps to solve for that (I needed different stamps since each roll was for a different value).
It turns out I cannot count correctly so I had to go to the post office a few additional times to buy additional small denominated stamps. I have had the sense that I have been doing the functional stamp equivalent of sawing the legs of a chair one at a time to get them all to the same length (and failing).
Having said that, I use stamps sufficiently infrequently that I wonder if I will have to add one or both of the stamp rolls (and the other loose stamps) for my kids to inherit.
I mention all this to explain to anyone who receives something in the mail from me as to why there will likely be two (or three) stamps affixed.