Well if they are so eager to be on the safe side because of "fuel quantity gauge error" and they add fuel to tanks because of it, why not be on the safe side regarding W&B and add "granny fuel" to loadsheet as well?
Yup. They also justify it by saying that the government regulating agency's pilots also use "granny fuel". Quite the claim and yet with zero evidence at all. Let's get really stupid - even if they did use granny fuel, they sure as heck aren't going to admit it in court to help support you when you're charged for breaking the law.
At the place I'm at it's company culture. The pilots don't know any better either - this is usually their first job. It gets them indoctrinated with this false logic from the start and then they eventually pass it on themselves. It's frustrating to explain this to them because they're essentially brain-washed into believing that their fuel quantity gauge is precisely 200 lbs over-reading (requiring 200 lbs of granny fuel - some use more) yet believe that the fuel totalizer is accurate.