Indeed. Avcat being heavier than Avtur in addition to, as has been already stated, the former's higher flashpoint combines to make it less efficient kilo for kilo.
Not sure why this would be a factor in the the fuel calculations for Black Buck though because only limited stocks of Avcat were held in Ascension and certainly nowhere near enough to have supplied the Victor and Vulcan forces (Avcat was stored for vertreps via APFC to RN vessels passing Asi).
The main fuel types at Asi were initially US-supplied F-40 (JP-4) Avtag and later F-34 (JP-8) Avtur. Question is, had the Black Buck missions been fuelled using F-40, then due to the fuel's lower SG and FP (0.75-0.80 and -18oC) compared to F-34 (0.77-0.84 and 38oC) would it have been more efficient than expected, given F-34 was the standard fuel used by the RAF at the time and by which I'm assuming all baseline figures would have been calculated?
My understanding is that aircraft that had been refuelled previously with AVTUR have to be completely defuelled when they embark and refuelled with the, higher flash point, AVCAT.
It is possible to work out a blend ratio in order to produce a composite of the two fuels which falls into an acceptable FP range for use on-board ships (ah, that was a fun lecture at West Moors) but from what I understand this is rarely done, navy types preferring to add any small amounts of F-34 brought on board to their F-76 stocks.