Hypothesis: he/she is some sort of known species with an odd voice. The "normal" whales don't recognize his/her calls, so they ignore him/her. They may even drive him/her away from the pod.
How's that for theorizing on the basis of nothing?
"Senator, when you took your oath of office, you placed your hand on the Bible and swore to uphold the Constitution. You did not place your hand on the Constitution and swear to uphold the Bible." -- Jamie Raskin
Outside of a dog, a book is a man's best friend. Inside of a dog, it's too dark to read. -- Groucho Marx
Maybe an outlaw spaceship bridge crew from the 24th century came and kidnapped a couple of whales back in the 1800's or earlier before we had whale tracking technology, and for some reason they decided to return one to our century.