Right, but in text mode the 9918 draws 6 native pixels for every 8 NTSC color "pixels". So, measured in NTSC color pixels, it's 320 wide (40x8) compared to 280 (40x7) for the Apple II.

This image (white text on a blue background) shows how the 9918's native pixels line up with the NTSC color burst frequency: https://imgur.com/rGcRpw0