What’s happening is something called “headspace”.
Essentially, the cartridge reservoir is built to be slightly larger than the volume. The bottom of the cartridge (the metal base) has a chamber that contains the ceramic heating element that vaporizes the oil in the cartridge and begins to absorb the oil right after being filled. While the cartridge is being used, the oil fills into the bottom atomizer chamber where the heating element is.
Because of this, the cartridge is primed to use right away after purchasing.
