Differences between laboratories. Different laboratories have varying expertise, technical methods, etc. Different timing of sampling. If some samples were collected 4-5 hours after ingestion, this would reflect peak plasma levels rather than trough levels. The presence of other medications.
Some medications can induce or inhibit VLP metabolism.
Tegretol is the usual culprit, since it strongly induces VLP metabolism Phenobarbital and clofibrate can also have this effect, but that doesn't seem to be a factor here. This should have very little clinical effect, however, since it is the absolute concentration of free VLP that affects for biological activity. Alterations in absorption from the GI tract due to functional bowel disturbance, such as diarrhea or constipation. I do not know how much this would affect VLP, and you may want to contact Abbott Pharmaceuticals to get their take on this entire question. I'd be interested to hear what happens.