Originally Posted by Wikipedia
The term PAL is often used informally to refer to a 625-line/50 Hz (576i
), television system, and to differentiate from a 525-line/60 Hz (480i
) NTSC system.
Source : http://en.wikipedia.org/wiki/PAL
So the maximum height (linewise) of the screen was different in PAL and NTSC. The MegaDrive had a 224 and a 240 line mode (http://en.wikipedia.org/wiki/MegaDrive#Video
). As you can see the 240 mode was only used for PAL games.