Because Intel cpu's are stronger on single thread performance than amd, and minecraft is a higher cpu usage game than gpu dependent. The g3258 is a dual core cpu that has such a low power requirement, that it can be easily overclocked and still not reach the power requirements of some quad/hexa/octo core cpus. OC on that particular pentium is the only way it keeps up with the stronger cpus, but being dual core it is limited to basically only single thread designed games like minecraft or skyrim, but really starts to suffer on multi-core games like Battlefield, FarCry, GTA-V etc. So while you will see good fps in a few games with the g3258, the rest will suck. This is where the amd A8 is better. It may be a little slower, it might have less thread performance per core, but it is not limited by thread count so is about equal in every game.
Those fps numbers were probably also from a g3258 that's been OC'd to @ 4.5GHz, and a corresponding igpu frequency boost and/or high speed/OC'd ram. You'll not see 60fps on an Intel igpu in cs:go any other way. Minecraft is also quirky, stock is easy to get super high fps, even at max settings, but add in some mega-mods and you'll see that number drop drastically as the cpu starts to get overloaded with data.