The third option is that Geekbench isn't crap in and of itself, but when one benchmark becomes overridingly important people will, of course, optimise for it. Geekbench isn't an important metric at all when it comes to marketing the speed of PCs, but is by far the biggest metric when it comes to marketing the speed of mobile devices.
You want a variety of benchmarks not just so that you get a varied picture on things, but so that this kind of thing never happens. Right now 3DMark counts for, maybe, at most 5% of the perceived speed of a GPU with the performance of real games accounting for the remaining 95%. Imagine if this was like the mobile device world, where a synthetic benchmark like 3DMark became all that anyone cared about. GPUs would quickly become much better at 3DMark, and would not advance as quickly in actual games as they otherwise would.
That said, the wide variance you see in GB based on OS does point towards the benchmark not being ideal in any case.