Yes, it appears a mistake was made on the Windfall map.
First of all, it appears an error of "1" in the 100 column was made when the subtotals were added up. The map printout says a total of 881 m, but the subtotals listed actually add up to 981 m (530 + 6 + 337 + 108 = 981).
Second, to double check where the error was made I took a ruler and estimated (as best I could) the lengths shown on the map printout:
Main Ramp (from end of portal) = 516 m approx.
F11 Ramp = 250 m approx
F11 Drift = 54 m approx
Total F11 Ramp and Drift = 304 m approx
F17 Drift = 120 m approx
Magazine = 6 m approx
Total ........................... = 946 m approx
Conclusion: The map itself is accurate. The error was made by the person adding up the numerical totals. The actual total progress made as of 15 August 2008 is 981 m.
Rek