Mac started off as Vindicator, felt bad about the way his first showing as a hero went against the X-Men, changed his name to Guardian.
Heather started off as Vindicator when she finally assumed a superpowered role, and eventually became Guardian.
Yeah, there’s no real good reason, and during the rare times both were active and alive at the same time they were either using both names, or interchangeably Guardian or…