I'll preface this by saying that I may have some misconceptions. Other people much more knowledgeable than I am have posted summaries of how modern graphics hardware works on HN before.

My understanding is that modern hardware is significantly more complicated at the lowest levels and (at least generally) no longer has a dedicated framebuffer (at least in the same sense that old hardware did).

My understanding of the memory access provided by fbdev is that it's an extremely simple API. In other words an outdated abstraction that's still useful so it's kept around.

An example of this complexity is video streams utilizing hardware accelerated decoding. Those often won't show up in screenshots, or if they do they might be out of sync or otherwise not quite what you saw on screen because the driver is attempting to construct a single cohesive snapshot for you where one never actually existed in the first place.

If I got anything wrong please let me know. My familiarity generally stops at the level of the various cross platform APIs (opengl, vulkan, opencl, etc).