Variable saved as string, not as integer
It probably has to do with how php handles POST values.

That is just generating a form element, which when the form is saved will get sent to the server as a string. Here's an attempt at phpfiddle to show you what's going on: http://phpfiddle.org/main/code/dmk-npy.

Basically you're either going to have to intercept the form submit and cast the variable to (int) there, or you'll need to cast it to an int when you're pulling the variable out.

Edit: Though I have no idea why your checkbox would be saving it as an integer.