'How I Met Your Mother' gets more viewers than 'Heroes'

Here are the numbers on some notable shows from last night:

"How I Met Your Mother" (CBS) - 9 million viewers; up 9% from previous week.
"Chuck" (NBC) - 5.8 million; down 15% [series low].
"Heroes" (NBC) - 8.2 million; down 11% [series low].
"Prison Break" (FOX) - 5.8 million; up slightly.
"Worst Week" (CBS) - 8.9 million; up 3%.

Now I can understand the huge dips in viewership for the hard-to-follow and often convoluted storylines that plague "Heroes," but what's up with the steady declines of "Chuck"? I'm a bit confused myself.