It could be someone using odometer tracking and that's their first entry. Fuelly doesn't calculate an individual MPG value in some cases
. So a combination of first fuel-up followed by partials could look pretty blank.
Keep in mind also that Fuelly calculates average MPG by using total distance driven and total fuel volume filled. It's not an average of calculated averages. So it's possible to have an average that is quite different from individual fuel-ups—especially if several partial fuel-ups are included.
What you could do when you see blank values is click on the "View All Fuel-ups" link under the Fuel Log to double check. That will show you odometer readings.