Which store? Doesn't really matter, just curious.

As to your situation, here's my best guess (having worked for lots of years for CT).
The sale was on CT's regular stock Mobil1, which you can identify because all regular oil has a 29-XXXX product number. The pack with the bonus was probably leftover from a previous special, and had a 99-XXXX product number (or vice versa - the point is, I'm guessing one was a special and one was regular stock). 99-XXXX products refer to stuff the company does not normally carry, and brought in specially for a flyer (usually a "no rainchecks" flyer item, too, since it's not normal stock).
The store paid more for one item than the other, so if they sold it to you at the sale price they'd be losing money.
All of that is obviously not apparent to a customer, who only sees two products labelled "Mobil1", and assumes they're the same thing. Should the store just eat the difference and sell you the more expensive product at the lower price? Maybe - that's not a black and white answer.
That said, their line about it being a "corporate thing" is made up. Individual stores can discount however they like (I believe the only requirement re: pricing is they can't price ABOVE what Corporate says, but I'm not ever 100% sure about that). However, working in retail for a long time, I'm not opposed to a white lie to justify store policy. In the end, it's the same point: policy is policy.