Why would my counts drop from normal to (rbc: 2.1, hct: 19, hgb: 6.5) after a 8% (400 ml) loss in spinal fusion. no internal bleeding. required 1 unit?

A few causes: First of all the amount of blood lost during a surgery is only an estimate. It is possible that it was somewhat greater. Second, you are often given a lot of IV fluid during a surgery and this can decrease the RBC counts by diluting the blood.

Some other cause: This can be anything from a bleeding stomach ulcer from the stress of surgery to sudden onset of hemolysis for any of a variety of reasons. Or your blood may have been switched with someone else's in the lab. Your physician can sort this out.