Those are swaths that the SRTM was unable to survey before its mission ended. There are only a few of them and luckily they're all inside the US where there is already higher quality elevation data available from the USGS to fill them in.
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
