Does your local oreillys actually carry all 4 SKUs though? So if you get to the store there might only be 1 or 2 of those choices available.
Pricing strangeness always occurs like this. Just look at the price of Soda in your local supermarket.
it's usually for 1-off small volume items like this. The buyers are price insensitive, picking up 1 quart of mobil1 for $11 because that's what they need; plus the volume of purchases are not high enough to make a difference.
Usually, the blue Stabil, the marine stuff, is the best buy. It treats twice as much gas per ounce so it goes twice as far. I now only purchase the small bottles because it lasts so long at the rate I use it.