Normal Map Storage (Not Unit Length)
Maps with z = 1.0 everywhere are likely partial derivative maps. This is a variant on normal maps that's become fairly widespread. There are a few reasons to prefer it, but probably the most important is it lets you store the map as just two channels (red and green), which allows using better quality BC5 compression, while being cheap to decode in the shader. The normals in such a map are of necessity not all unit length, and as you pointed out that will affect interpolation and blending slightly, but this is generally deemed an acceptable trade-off.

However, if it's purporting to be a traditional old-school normal map and it has pixels that are significantly off from unit length, that sounds odd and probably unintended. I'm not sure why that would have happened in the particular Wikimedia .png example you linked. However, non-unit-length normals could generally be introduced by doing filtering/blending/compositing operations on the image without renormalizing after (for example, if a normal map was rendered at a higher resolution and then downsampled using a standard image resampling algorithm). They could also be introduced by lossy compression.