What I don't understand is why TAR 14's double-length leg isn't counted as 2 legs?
There's a variety of definitions people use for distinguishing legs like this.
One way to split is to separate when there's a "To Be Continued". This started on Season 6, but teams didn't visit Phil at this point- the episode just ended with teams still racing, waiting for a shop to open. In the next season a clue sent racers to Phil, where he said "The leg is not over, here is your next clue!" (a double-length leg). Season 10 had something else funky, in that teams were never told to go to a pitstop or see Phil, they just opened a clue that said "You're still racing!!!!!", and I always thought that was kind of strange. I believe Season 18 (Unfinished Business) was the first "To be continued" for Phil to say something like "Your next leg begins now, here's your next clue!" (a zero-hour pitstop in the middle of two legs).
Another point is if they got a prize. Before Season 18 teams didn't receive a prize for showing up first at these locations, making it more obvious that it is the midpoint of two separate legs. However, Jamie & Cara actually did
in fact win a prize, but it wasn't shown, but then again Phil said that the leg wasn't over.
For his handy list of team average placements, I'm pretty sure theschnauzers counts any visit to Phil as a separate leg, and he might also count Seasons 6 and 10 as two legs despite teams not ever meeting Phil at a midpoint. For my list of average placements, the way I separate legs is if Phil says that it's two legs rather than a continuation of a leg. I wouldn't be surprised if that's how the show did it, if it even cares about that.
Honestly, TAR14 is more like the recent TBCs and should probably be counted as two separate legs, but because it was the first to be like that the show just used the old jargon. I probably gave a lot of mostly unneeded info, and I probably didn't even answer your question.