Can I clarify, did you extract the basin for a specific point that you digitized? If so, did you use the Snap Pour Point tool to reposition your digitized point onto the 'digital' stream (i.e. the path of high flow accumulation derived from the DEM) as per the instructions given by ESRI? If so, this is likely your problem. The extraction of a watershed (or basin, or catchment area, depending on your terminology and location) is highly sensitive to the positioning of outlet points. We use tools like Snap Pour Points so that we don't get 'stub watersheds' (much smaller watersheds than expected) when our outlet point falls off the digital stream. Moving that point by even a single grid cell can result in the mapping of a completely different watershed than the intended. Unfortunately, when the outlet point is positioned near a confluence in the river network, application of Snap Pour Points is very likely to result in a repositioning that will yield a completely incorrect watershed. I talk about this phenomenon in my answer here and a blog here. Ultimately, however, I recommend not using Snap Pour Points and instead use an alternative algorithm developed by Jenson to reposition outlets. Hopefully this is of help for you.