1st generation = 50% human, 50% demon/devil/night hag, or simply fiend. This is a half-fiend, or a halfbreed if you will.
2dn generation = 75% human, 25% fiend. Not a halfbreed any more, as you can see. More like a quarterbreed.
3rd generation = 87.5% human, 12.5% fiend.
4th generation = 93.75% human, 6.25% fiend.
5th generation = 96.875% human, 3.125% fiend.
There. It should be fairly obvious now that a tiefling only five generations removed from its fiendish ancestor is decidedly more human than you would suggest. Tieflings aren't half-fiends, they just carry (according to Races of Faerûn) "the taint of evil in their very souls". The math proves it. 3.125% isn't enough to warrant a half breed title, but it is enough to call a taint.
Tieflings shouldn't be put in the same cathegory as half-orcs and half-drow, who indeed are 50% human and 50% something else.
EDIT: Indeed, the pictures are quite snazzy.









