My DD also has what appear to be odd broad score calculations. I have wondered the same question as the OP for a while and was not able to find any answers. It seem odd to me (and I am certainly not a mathematician) that the Broad RPI score is lower then any of the RPI's in the subset. I figured it had to be something with not having a representative sample of scores.

22B - my W scores for the broad scores are indeed the average of the W scores for the subset.
FWIW

Letter Word ID RAW 40, W 467, RPI 161 SS 100/90
Reading Fluency RAW 21, W 482, RPI 142 SS 98/90
Passage Comp RAW 19, W 469, RPI 140 SS 100/90

Broad Reading W 473, RPI 134, SS 100/90