given IELTS round their score up to .5 and .0,
7.5 6.5 6.5 6.5 is mathematically 6.75, which is 7.0 for IELTS, i.e. C1.
if one has something like 7.0 6.5 6.5 6.5, which is 6.625, then it will become 6.5 for IELTS, i.e. C2
the median of B2 is 6.0, the median of C1 is 7.5
so, the boarder line between B2 and C1 = (6.0 + 7.5) / 2 = 6.75, instead of 6.5
P.S. indeed, not much people care about B2/C1 for English. CEFR is for various European language more in practise. People just use IELTS / TOFEL score for English in whatever application purpose (either school or visa...)
The purpose of comparison is just to have a feel about how difficult France B2 is, and thus the definition of border line doesn't need a precise definition from user point of view....