LOL, it took me a long time to make this in photoshop. ah well i might as well post it:
here is what has happened. after block 478558 on August 1 bitcoin cash (BCH) fork appeared. and their blocks started building on top of #478558
now these two chains are separate from each other but they both recognize the old blocks. this means the BCH chain and BTC chain both recognize any block smaller and equal to 478558 but will only recognize blocks on their own chain after that point.
for example this means any coin on block #478556 is valid on both BTC and BCH.
but a coin on #478561 is only valid on one of these chains.