BiS exists but is not mandatory. It is not relative. Just because somebody performs better with a different weapon than the stated BiS does not change meaning of the phrase. Honestly how is it difficult to understand this?
Uptime and staying alive is typically more important for higher overall damage output than raw numbers. A weapon that may give you the ability to maintain uptime without having to heal or prevents you from dying may be better for YOU in that specific situation but that doesn't make it Best in Slot. The concept is based around optimal play which is very rarely, if ever, even achieved.
If something works for you go nuts but don't expect other people not to question it. Tell them to piss off if you need but that doesn't mean the concept of optimisation isn't objective.