Yeah, a lot of that is driving habits. Even bone stock (i.e., rich), if you are reasonable, you should be seeing almost 24 mpg combined.

 

My wife does accounting for a large gas supplier in the Northwest.

They do some very questionable things. Such as if they order 1000 gallons of regular, and the regular tank fills, they have to put it somewhere. Which usually means going into a plus or premium tank. I've found that mixing is quite common.

More fun info, fuel is extremely temperature sensitive, and can expand and contract in size easily from it. 9 times out of 10, you aren't getting the exact amount you bought. ie 10 gallons != 10 gallons. It can be + or -, but usually -.

 

^^ its less if the ambient temperatures is over 60 degrees (that's the fixed temp its measured at during distribution). In 2009 there was a class action case against a bunch of major refuelers because the pumps didn't have temperature adjustment built-in. Since then, the temp is factored in. Costco was one of the ones sued, and iirc they gave all their customers a 10% off-your next fill-up coupon or something if they settled.

I used to work in litigations and did some work where small independent chains were getting screwed by the big distributors. Basically, the major distributors would sell their gas to the nobody chains for $1/gal, but sell to stations with their name-brand for $0.95/gal. IE: Citgo sells to Citgo stations cheaper than they sell to Hess/GetGo/Sheetz/etc. That gave the brand stations a 5% advantage that they were able to use to try to force competition out of business. pretty shady stuff!