How is the Pal system different?
I haven't tested Flapee Bird or Princess Quest on PAL yet -- just arrived today -- yay!
But, to your point:
On NTSC systems, the video updates 60 times per second. On PAL systems, this happens 50 times per second. Since most games use this interrupt to 'time' things and update the position of objects on the screen, etc., unless they account for this discrepancy, games on PAL will seem to run more slowly. Ironic, since the CPU is actually running a bit faster on PAL systems.
The easiest way to think of this is:
How would you 'time' one second on an Intellivision?
There's not a simple way to do this, but, you do "know" that this video update interrupt will happen regularly.
Well, if you increased a value 'n' once every time you updated the screen, then you know every time you get to 60, one second of time has passed. After all, if this didn't happen pretty close to exactly 60 times per second, you wouldn't get a picture on the screen.
So, if you write your program to update your 'number of seconds' by one every time your value of 'n' hits 60, you're set.
Now, take that same program and run it on a PAL system.
Oops… On PAL, if you still wait for 60 'ticks' to pass, but it takes only 50 for a "real" second to pass, then you're going to wait 10 'ticks' too many. Therefore, your clock will seem to run too slowly.