You are refrring to 7th bit of mac address the U/L bit not the 7th bit of IPv6 address I guess :-)
You need to look at bit side of things. It makes thing easier in notation.
It's described here:
Take a very simple MAC address and convert it to EUI-64 with and without the bit flipped.
You will see that it's "easier" to write it down when it's flipped.
This is explained in RFC4291 Section 2.5.1. In short, it is so that the implicit modified EUI-64 portion of manually assigned addresses (such as 2001:db8::1) will be recognized as locally administered.
The motivation for inverting the "u" bit when forming an interface identifier is to make it easy for system administrators to hand configure non-global identifiers when hardware tokens are not available. This is expected to be the case for serial links and tunnel end-points, for example. The alternative would have been for these to be of the form 0200:0:0:1, 0200:0:0:2, etc., instead of the much simpler 0:0:0:1, 0:0:0:2, etc.