Why does the store say a game is $11.99 and then charge $29.99?

The sales have posted start and end dates. They post the lists weekly on a variety of social media as well as Xbox

What game are you looking at?

That will also sometimes happen if you're looking at a game in a different region and then when you purchase it switches to your own as prices can very from region to region.

You have some legal issues there. It is not incumbent on the "receiver" of the advertising to determine its veracity unless an expiration date is clearly denoted on the advertisement. Nowhere on your advertisement does it list an expiration date. Basically,
what you are telling me is that this listing is like an expired coupon. Unfortunately, you did not put an expiration date on the "coupon."

Michigan would be US.

It looks like the store on your console is still showing the sale from last week (it ended on the 13th). That game is no longer on sale. You may want to try a power cycle to see if it'll update the store correctly (hold the power button down on the console
until it turns off, unplug it, plug it back in, then turn it back on).

Sniper Elite 3 under "Deals" says it is $11.99 but, when you click on it, it says it is $39.99. I am in Michigan. I don't know what region that is.