It finds the closest entry in there to the current value of "new Date().getTime()". Then it just adds (what roughs out to) .000004 for every second's difference between the 0 value and the current getTime() value. So today it reads 10250 MB, which must come from index 2 of that array + enough seconds to add 10 MB and some. Definitely more legit than Santorum's.
Here's the code for the counter (it's embedded in that HTML file) http://pastebin.com/8tebjT79
The actual subtractions of the current time from the one in the array seem to be more detailed than what's put on the page, which is being rounded. A sample of input for format():