The corn used in lab blocks is a higher grade than that used in most animal food. This is because lab blocks are used in research labs and scientific institutions and the blocks must be formulated exactly for experiments and such. There are also a few different manufacturers of lab blocks. I recommend and use Mazuri brand. What I do is, the day I clean cages I feed only lab block for a day or two, then add the other foods. This way the mice are forced to eat the (nutritionally complete) lab block at least a couple days so that they won't ignore it and go for the less complete stuff like millet or sunflower seeds. It's the equivalent of making a kid eat his brussels sprouts before he gets a doughnut!
The corn itself it responsible for increasing the rates of cancer in meeces due to a reaction between stomach acids and corn, which forms the carcinogenic substance. (I think it's nitrosamines; it's been awhile since read that article and the site is no longer up.)