Yes
"Are the indices on page 375 of BJA3 for 6 deck?"
Yes.
"If so, why are there some differences from the 6 & 8 deck indices on page 213, such as 16 v 9, 12 v 2, 12 v 5?"
Because the former were generated by John Auston, using Karel Janecek's SBA, and the latter by Norm Wattenberger, using CV.
The three inidices that you quote, not surprisingly, are notorious "in-betweeners," know to us for a long time, as having precise values, to the tenth of a decimal point, very close to half-way between the two different indices that are given on the two pages you quote.
Don