The other day I had to run to the market across the street for some sugar. I was out and didn't realize it, and needed sugar to make well, a lot of stuff! So of course I gravitated to the store brand sugar, figuring it didn't really make a difference! But then the name brand caught my eye...and more specifically the price tag. No idea why, but name brand was actually 30 CENTS CHEAPER than store brand. Yeah, crazy. I went with name brand.
You may also find illogical pricing like this when comparing sizes. Normally you'd think the large box of waffles was cheaper per waffle than the smaller boxes. This also used to be untrue at our store. The mid-size box actually saved a few cents when comparing unit price!
Finally a reason to check is some stores may have policies in place to insure their "correctness." A local grocer will give you an item free, up to $10, if the price rings up incorrectly. Once I got a free $10 pack of toilet paper because of this! In fact, it was happening so often I was worried the store manager was going to thing I was tricking them! Honestly I just remember advertised prices.
So keep your eyes sharp and save a few cents!