MFP's Circuit Training Calories - is it a mistake?

meghan1789

So normally I only do my 30DS video once and I log it as 20 minutes circuit training (I don't have a HRM). But today I did it twice, so I went to log it as 40 minutes and I noticed something funny.

20 minutes = 237 calories
40 minutes = 438 calories

Now math was never my best subject, but that doesn't add up. Shouldn't more double the time mean double calories? Or is there something I am missing?