Solved! Go to Solution.
Here is a nice explanation provided by Tom Vanwalleghem to a similar thread on the GIS Stack Exchange.
"The units of upslope contributing area are m2/m or m (in fact A is As or specific contributing area, contributing area per unit contour width) So that's why you multiply by cell resolution = 30 m and not by cell area (30*30) A more correct way to calculate this would be to take into account the flow direction (if it is diagonal or perpendicular), but for most applications, the approximated calculus given here is enough."