Does anyone know under what circumstances fscommand("save","file.txt"); saves the __proto__ and construct variables?

Further explaination:
I'm using this (now outlawed by MM) undocumented function to save XML objects as strings. The problem is that it's saving the __proto__ and construct Flash environment variables along with the string I want it to save and they screw up the re-constructor as well as make my file sizes slightly bigger.
I can delete them when the file is loaded up later on but that's a work around and I'd rather know what causes the issue.
The variables I wish to save are set up on _root just before the save command is run. The app is quite complicated but when I make a simple replica with less code it doesn't happen so I'm stuck...