Why does 'select within a distance' produce a differrent result than 'buffer/clip'? Shouldn't these 2 functions produce the same result? Red circles indicate date that should be in the result. All shape files and data frame are in the same projection.

REMYSYLVE

IanMurray

I would guess it would have something to do with the projection you are using, and whether you used a planar or Geodesic Buffer when you used the Buffer tool.  If you are working with a geographic coordinate systems instead of a projected coordinate system, it does not take into account the curvature of the earth when calculating distances with the select by location(within a distance).

http://gis.stackexchange.com/questions/131766/why-the-select-by-location-analysis-calculates-distance-in-a-wrong-way