I have four named characters in my White Forest: Britney, Collin, Gene and Vincent. According to Bulbapedia, this means that in patches of long grass I should encounter Hoppip, Abra, Aron and Magby, which has proven to be the case. However, I should also (by Bulbapedia's reckoning) see Surskit, Lotad and Corphish while Surfing.
I've had about 30 encounters Surfing so far, and most of them were Surskit, with a handful being Lotad. I haven't yet run into Corphish. The Bulbapedia page seems to suggest that each of these pokemon should appear equally frequently, but my experience (while I haven't gone through the whole song and dance of a statistical test) doesn't seem to bear that out. Even if you interpret it to mean that Surskit should appear twice as often as the other two (since his presence is due to both Britney and Gene), it still doesn't seem to be consistent with the results I get.
In the case of tall grass encounters, too, it seemed to me that I saw rather more Hoppip and Aron than I saw Abra and Magby. In both tall grass and Surfing encounters, it doesn't seem to me that each of the possible pokemon is equally likely. What exactly decides how frequently each pokemon appears?