Grading rules have changed a bit over the years, but your contractor might be correct. Further, it is possible you have a full size rather than nominal size joist (2x12 was actually 2"x12").

Find the prevailing grading agency for your area and see if you can get some historical data.  The National Forest Products Association might be of some assistance.

The joist is obviously performing if you don't notice excessive deflection or creep, making it stronger than your calculation imply.

If the joist still has sawmarks on it, it may be full size. I don't know Ron's age, but 50 years ago, in Canada, a 2x12 was 1-5/8x11-5/8 and the lumber was seasoned (shrinkage was minimal). Typical Doug Fir was either #1 or Select and the flexural strength for #1 was 1500f and shear was 120 psi, if memory serves. These values were based on service loasds, not factored loads.

dik...I'm a little older than the project!  In the US at that time, we had nominal sizing as you've noted for Canada; however, there were still many locally produced lumber items that were rough cut and sized.

There have been many posts on this subject, I suggest you google it at the top of the page.