Sometimes when people meet your new girlfriend you overhear them saying "How the hell did that ugly sack of shit land a cool dude like that?! I can't get my head around that!" There's a reason you can't "Wrap your head around it" because that's physically impossible unless you're Gumby or Mr. Fantastic. I don't know if you've realized this, but your head is comprised of dense skull bone, eyeballs, and face meat. That's not exactly what they make rubber bands and slap wristbands out of, for a reason! Of all the things that comprise a human body, your head is probably the least flexible thing that can't wrap around anything. When someone tells me they can't wrap their head around something it makes me want to wrap their head around the nearest telephone pole.
Even if you were trying to say "I can't wrap my mind around that" this statement still doesn't make sense. Who says you have to encircle a concept with your thoughts in order to process it mentally? Your mind isn't a group of covered wagons surrounding a buffalo. When you're trying to understand something all you have to do is think about it rationally and either it makes sense or it doesn't. I'm not trying to wrap my mind around why people say they can't wrap their heads around things, I'm just dissecting it and analyzing the structure of the saying and pointing out that it's ridiculous and people should start wrapping their minds around the shit they say before they start wrapping their heads around what other people are saying.