Two cents goes a lot further than it used to.

It's called marketing. They assume people will just see the $39.99 and not pay attention to the 3 payments part. And I'm guessing that it works quite a bit.
Either that or they know that most people's math skills are pretty bad.

Or it could just be Bud's answer which is better than mine.

Because math is hard.

At least, the advertisers know it is for a majority of their viewers.

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