ArcMap "Select by Location" returns different results than Geocommons analysis - why?

Discussion created by RenataPSilberblatt on Apr 9, 2013

When I run a Select by Location query in ArcMap 10, I get a certain number of features but when I run the exact same query in Geocommons (Filter by Distance), using the same files and the same distance, the analysis returns a different number of attributes.

This doesn't happen with all the Select by Location/Filter by Distance queries I've run (sometimes both programs have matching results) but it's happened a number of times.

Any ideas as to why this may be?