I am pretty OCD and I hate to see the red flashing number telling me I have no free settlers. So, I arrange things so that when my army is fully filled out (7350 units) and all the mines are built including the bonus iron one from my skilled geologists, I have 12 Settlers spare at my pop limit of 7700. Recently I noticed that this number had changed despite everything being full, I only had 11. The next day I only had 10 and the day after that it went down to 9. I hadn't placed any new buildings in that time. Tonight, I placed a new Gold tower and as expected the number of free settlers has changed. Except that it's gone up. I now have 14 free after everything is placed. My island is pretty carefully tuned so adding or removing a new building isn't something that happens without me paying attention.
What gives?