odd bc -l output

[Peter J. Philipp]

In fact it's not just bc -l, but also when I calculate the following in C
(linked with -lm)

                C = (180.0 - A) - B;
                a = (double)(c / sin(C)) * sin(A);
                b = (double)(c / sin(C)) * sin(B);

Some may recognize this as parts of the Law of Sines.

pjp@neptune$ bc -l
(9 / s(70)) * s(76)
6.58357679385302895866

When I do it with xcalc I get the correct 9.2931043.

What am I doing wrong?  How must I massage my system the correct way?
The wrong number was observed on arm64 (bc -l) and amd64 (CGI).

Best Regards,
-peter