FFT of white noise with different sampling rate
The variance of sampled white noise that is filtered by a brick-wall lowpass filter is reduced by a factor equal to the ratio of the filter cutoff frequency to 1/2 the sample-rate.  You start off with the same variance for the 100mhz and 10mhz cases, but the reduction factors are different due to the different sample-rates. Or to put it another way, the 100mhz-sampled signal has less power/Hz than the 10mhz sampled signal, so when you compare them over the same bandwidth, the filtered 100mhz signal has less power.