Part of this issue is more of a modern viewpoint. In the 80s and 90s hair bands, for example, were considered metal without question. Younger people now don't think of such bands as metal because they don't view them as heavy enough as modern bands have pushed heaviness to its limits.
So that is likely at least part of the discrepancy in definition.
So that is likely at least part of the discrepancy in definition.
I generally consider metal as having intensely distorted guitars instead of the 60s/70s stuff that is more of like overdrive (turn a guitar amp up so loud it naturally distorts the sound of it). They have actually created pedals and effects processing that recreates overdrive without the excessive volume.