There is also a far greater number of Mythic guilds that raid at their own pace and don't participate in the rankings race. You will see number shoot up over the next 2 weeks.
Most normal players with social lives, responsibilities, etc. won't be starting Mythic fights for quite a while. The standard gear progression goes Heroic>Mythic. Some people would even argue that its more like LFR>Normal>Heroic>Mythic. Most players don't have the gold, gear, or time to jump straight into Mythic. Why make something harder for yourself?
I imagine this number will go up quite a bit today, it'll be the first chance for many guild to try. My guild used Tuesday to clear normals and yesterday for heroics. Tonight we're going to give M Kargath our first try and he doesn't look that hard when compared to most others.