Generally speaking, in order to fit the GPU portion on the chip you need to find more room to do so. In the case of the "G" series, this often results in the processor being reduced in some way. With the 5700G, it has half the L3 cache and only offers PCI-E 3.0 support over PCI-E 4.0. Some of the factors of cutting the chips down can result in weaker performance, considerably so in some applications. That's fine if you're buying a "G/APU" CPU for light tasks, but the 5700 when following the naming scheme AMD use implies that it's a cut down 5700X when in fact it's a 5700G without the GPU. I'm guessing AMD has a bunch where the GPU portion just flat out failed during manufacturing and decided to disable that portion and release them under a different name. Goes back to my earlier comment about AMD having horrendous naming schemes in general, although Intel have been just as guilty of it in the past. As far as I'm aware, the "5700" is the only part of its type, and hopefully the last unless AMD get their **** together and advertise it properly.
Speaking of Intel, the "K" segment used to mean that the CPU had an unlocked multiplier and could be easily/heavily overclocked. With the way current CPU's function that's not much of a thing anymore.