I thought you'd been a fan long enough to know this one?
Anyway, what it is is that in real life here on Earth they made a backstory for Boba that was that he was Jaster Mareel, a Journeyman Protector from Concord Dawn who was exiled.
In-Galaxy the Jaster Mareel story is a rumor that was started because Boba and Jaster wear similar armor, and it's a rumor Boba likes having, among the many other rumors about him. Jaster really was a Journeyman Protector, really was exiled, but became the leader of the Mandalorians. When the Mandalorians split into two groups, Mandos and Death Watch, they were chasing each other around Concord Dawn and happened upon the Fett farm about the time Jango was 10. The farm burned and Jango's family died. Jaster felt bad and basically adopted Jango. So in a way Jaster is Boba's grandpa.
There's a way of thinking that we're researching the SW galaxy and all the books are written based on what the researchers know of the galaxy and in the last several years we came across a lot of new information. So the original stories that have Boba and Jaster as the same person were written based on the rumors that we now know to be false. If that makes sense?