The reason is that the sim count only applies to Sims flagged as active. If you go into the Manage Households interface while in manage worlds, you can move sims back to "Other Households" after you make your desired edits to them. Just click the Heart symbol on them.
BTW, the maximum sim count you're talking about is not the total number of Sims the game will allow there to be, it's just the maximum number which you can protect from culling by having them marked as active as one of your households. Culling still happens either way and there's no way (without mods) to adjust the thresholds for that at all.
Just as another FYI, if you are willing to use mods, the MCCC mod lets you modify the outfits for other sims without needing to make them active/played just by clicking on them anywhere you see them out and about.
