Personally, I've always thought it kind of improbable that each elf has a perfect match out there. Rather, it seemed to make sense that recognition happens between two elves who are a pretty darned good match... and that the act of recognizing them binds them irrevocably. And because of that bond, they don't go out and recognize anyone else- even if one of them may someday stumble upon another elf who is also a pretty darned good match.
I think that's why, when two tribes meet up, you get recgnitions. Because the gene pool suddenly has a chance to diversify- and that's a good thing. So the two (or more) best candidates match up- and bam.