[nraynaud]

nope it's the SRTM itself. It's a quite low quality data, or more exactly, it's not meant to be used directly, you theoretically need some post-precessing and cross check of this data before use.

Of course normal people don't have the means to do that so we use them directly. I have not personally checked it, but this guys offers something that seems higher quality than the raw SRTM (in particular in the mountains): http://www.viewfinderpanoramas.org/dem3.html