Problem with Mersenner Twister

I try to use Mersenne Twister in the SDK to generate random variables. I just do a simple test to compute the mean and the variance of GPU output. But I cannot get the expected results of variance.
For example, after BM transform, the variance is around 0.945 rather than 1. And before BM transform, the variance is around 0.074 rather than 1/12. Does anyone have any idea about this?