When I hover over my picture in recent reviews, it says I have reviewed 147 first posts this week, however there is a limit to 20 reviews per day, and last time I checked, there are only 7 days in a week, so the max I should be able to review in a week is 140. So it seems like it is a bug that I was able to review 147 first posts (not that I'm complaining, it helps me get closer to the steward badge for the queue).
Indeed, there're only seven days in a typical week, you aren't mistaken.
The skipped review tasks don't count either (so weird it would be otherwise). The review audits do count towards daily reviewing limits and towards the badge.
Most possibly, the reason is the ambiguous terminology regarding time spans. As you may know, Stack Exchange considers an event to have happened "yesterday", if it has happened 24 to 47 hours 59 mins ago. So technically, if something has happened 40 hours ago, it's still "yesterday". Same goes for "two days ago".
So I do believe that that specific "week" (not to confuse with the actual calendar, i.e. Sunday to Saturday, week which is used for example in the reputation leagues) is defined as "not before than 24*7+23:59" hours ago, and during those eight days, you could have definitely reviewed 147 tasks.