User blog comment:FairyRaven/Fairy Tail Chapter 392: You Must Not Forget, Review and Commentary/@comment-24800928-20140711144951/@comment-7418318-20140711151935

Could be. And with all the research that Ivan did against Fairy Tail, he would surely have made precautions against Natsu's sense of smell to make sure his Alexei disguise wouldn't be so easily exposed.

Or the smell could be something that might be similar between family members, but doesn't have to be.

Something like: If two people smell the same, it proofs (or at least strongly hints) they are related closely, but if they don't the reverse isn't necessarily true.