Would it be possible for Larcade to be the prototype for reviving Natsu, which ended in a failure until waaaay later after Zeref met Mavis, who somehow made Larcade perfect or made him successful? Might explain Larcade smelling like Natsu - clone of him or has his DNA or some shit, and Mavis is the mom because she contributed to Larcade being brought to live or revived successfully. Zeref would be the dad because he either raised Larcade or that and he created Larcade as well.
One very unlikely explanation is that Mavis and Natsu are somehow related to each other, which makes no sense at all. Assuming it's possible for Mavis to still the same smell, Natsu so far was never older than a kid before Mavis's time, so he's too young to have been her descendant. Unless she went back in time and played a part in Natsu's resurrection and Larcade's birth, which is possible. Could have happened before Zeref desouled her or a possible future after te war.
Or maybe Sting hasn't smelled Zeref before, so he doesn't get that some Dragneels or some people can smell alike for whatever reason.