Maybe because he got the points first therefore placing him first and the you tied with him, but since he git the high score first, then he get 1st place.
Kind of like the rankings in Strong Approaching, wherein it doesn't matter if you are tie in score, but so long as someone got the high score before you did, they get to be first place.

as far as i know Nature Training ranking does not depend on who finished first the challenge

it depends on how fast (or how early in the round) you finished the challenge, for example Level 50

lets say Player A finished the challenge in 4 rounds first, he will get the 1st ranking

then Player B finished the challenge in just rounds, so Player be will take the 1st ranking

i've tested with friends, initially i got 1st rank.. then he who had double of my BP finished the challenge & take my rank

unlike Strong Approaching, like you said.. it depends on who finished the challenge first

Hello,
Shumihaza is correct; there is a time component to this. Whoever finishes it faster will receive a higher placement among players of the same score.
Thanks and have a great day!