Ah, ok. Thanks Golden Shaymin. I just translated part of the faq page on the Advance Map site and it seems to confirm what you wrote. Thanks for the info.
EDIT: A follow-up question. For fishing pokemon is there a method to determine which pokemon are caught by which rods?
EDIT 2: Something doesn't seem to add up here.
Example: Ruby Route 121 Grass
1. Zigzagoon
2. Duskull
3. Linoone
4. Duskull
5. Linoone
6. Oddish
7. Oddish
8. Gloom
9. Wingull
10. Wingull
11. Wingull
12. Kecleon
So that should equate to: Zigzagoon 8.3%, Duskull 16.6%, Linoone 16.6%, Oddish 16.6%, Gloom 8.3%, Wingull 25%, Kecleon 8.3%
Compare that to Route 121 on Bulbapedia.
I'm sure the game is doing some rounding, but is there more logic involved? Or is bulbapedia just plain wrong?