How is "average days/week" calculated in the statistics page?

Hi Blenders!

Since joining FB Plus I've been enjoying checking out my statistics, but often times I find the number of average days / week shown in the page smaller than it actually is. I'm very confused about how it is counted!

For example, today it reads 4.82 days/week in the last 30 days, but attached is what my actual 30 days looked like. Based on this there were 23 active days in 30 days and it translates to 5.37 average days per week @_@

Anyone please help me understand!