Trail runner said: "A properly fitted pack will make your load FEEL lighter and an ill fitted pack will make your load feel heavier."
While I don't doubt this is true, I think it's also true that "An actually lighter pack will make your pack actually lighter".
Seriously, IMO if the overall pack weight gets low enough, a person worries less about the pack fitting properly, or spending a lot of time trying different packs to get the ideal fit, and adjusting it to get the fit properly "dialed in".
This no doubt involves some individual preference, but I think it also has to do with the overall weight being carried. With a base weight in the low teens, I'm content to buy a lightweight pack via the internet; I've purchased two different models (from two different manufacturers) this way and was not disappointed in either case.