Maybe purchasers from Asia and Europe weigh more in the average at that time (due to U.S. being asleep), and they spend less money?
What's the stdev for that time period? Are you sure it's not just a few outliers pulling down the average?
We checked the raw data and it all clusters around $75 during that time period for some reason. Items on Fab.com range from $10 to $5000, with thousands of purchases captured in that data.