Some vendors have a buggy implementation of uncommon or advanced features. For example, at this time, very few developers are using imaging extensions (convolution, histogram,...) so some vendors have a higher priority in fixing bugs in more commonly used features.

In the very worst case, you may even find drivers that claim to support GL_ARB_imaging in the extensions string, but with non-operating functions, that may seem to do nothing, or even garbage the OpenGL state in some situations.

If you need imaging extensions, you're urged to download the latest drivers from your vendor, or use veteran graphics workstations, which have been supporting these features since the mid-90's. Or, alternatively, the Mesa 4.0 software implementation has a great support of GL_ARB_imaging.