Unexpected result calculating the determinant of a singular matrix (42S)

[Dave Britten]

When calculating the determinant of the matrix [[-2,1,3][1,2,1][3,1,-2]] on my 42S, I would expect to get 0 as it is a singular matrix. But the 42S says it's 3.30000000001E-12. How does the 42S calculate determinants that would lead to that result? And how do I identify when it's giving me a suspicious result? I've done very little linear algebra, so there might be something simple and obvious going on here.