I agree that it's very confusing. The instructions for each product are identical and state to add the entire contents of the bottle to "up to" 21 gallons of fuel. So whether you have a 6 oz. bottle or a 12 oz. bottle, you're instructed to use the entire contents per 21 gallons of fuel.
I would disregard what the e-mail claims and take this at face value. I would assume that the same amount of active ingredient is incorporated into each bottle of Regane and use it as directed.
Milk no matter the container size is going to be the same. If both the 6oz and 12oz Regane treat the same amount of fuel, one would think the 6oz may have more concentrated ingredients. I’m just trying to find out which is the better product to use in terms of cleaning and value.
Depending who you wish to believe, the evidence in this thread below suggests that both bottles contain the same amount of active ingredient. The smaller bottle is more concentrated. It's suggested that this is due to retailer marketing.
In the Gumout FAQ section they imply to follow the bottle directions for proper dosing. That email answer above confounds any logic, because it suggests that the two bottles contain the same ingredient % concentration. WOW.
I was under the impression that the amount of PEA was the same in both the 6oz. (Walmart) and 12oz. (AZ) product and that using half of the 12oz. bottle would give you half the PEA for treatment. OTOH the larger 'All in One' bottle claimed to treat 35 gallons of gas IIRC...and I would often use half a bottle twice in my 12 gallon tank.