Why does IETF use a non-binary notation to enumerate bits in HEADERS?

Hi,

​

So, I'm using more RFC's mainly to get more precise info about protocols and to get some PDU's to post on a networking blog I'm working on and I got this in question.

​

Why does IETF use a non-binary enumeration to bits in headers and why it doesn't enumerate mainly bits from 0 to 7 in Headers? Why it counts bits from 0 to 9 and count it in a decimal notation? 0 to 3 above bits

Example. https://imgur.com/27YGG6u