Hi Tony Ellis,
I just made some changes, you are right this is confusing. In RFC5059 they state it like this:
BSR Priority
Contains the BSR priority value of the included BSR. This field
is considered as a high-order byte when comparing BSR addresses.
BSRs should by default set this field to 64. Note that for
historical reasons, the highest BSR priority is 255 (the higher
the better), whereas the highest RP Priority (see below) is 0
(the lower the better).
They talk about highest priority for BSR and RP selection, but for BSR highest priority means the highest value (255) and for RP, it means lowest value (0).
Rene