At first I misunderstood this stat. But aparently it is teh rate that other teams get first downs when they have the football. According to my calculation Dallas has given up the 2nd fewest first downs in the league behind Cleveland. But Dallas and Cleveland have on 8 games vs 9 for New Orleans and Baltimore. This stat is a measurement of the number of 1st downs per set of downs so the number of games is irrelevant.
This is an impressive stat but the 5-3 record shows the Cowboys have had some really good games and some not so good games. The Eagles had 20 first downs against Dallas, 10 passes, 7 rushes, and 3 penalty. The problem for Dallas is most of those 1st downs came on 4 TD drives - which shows that first downs is less important than rate of scoring drives allowed.