Anyone know what causes fluctuations in the Cornell multiplex Lyme test results in a horse that I would think everyone can agree is not infected per test results?

Here is my scenario:

Jan. 2012 test results:

OspA Value: 119 Negative
OspC Value: 48 Negative
OspF Value: 136 Negative

End of May 2012 test results:

OspA Value: 481 Negative
OspC Value: 17 Negative
OspF Value: 100 Negative

As I understand it, OspA Value is supposed to show whether a horse is Lyme-vaccinated (but levels can also change during infection). OspC Value is supposed to show early infection. OspF Value is supposed to show chronic infection.

All of his levels on both the Jan and May 2012 tests show levels that are well below positive. Nonetheless, as a point of curiosity, I am wondering what would account for the fluctuations. Particularly in the OspA Value in a horse that I know for sure is not vaccinated. Could it be that he has been exposed and fought it off? Something else? Very curious about how this all works if anyone knows. Based on the results, I'm confident he does not have Lyme. But I still do wonder about the fluctuations and what they mean. I will ask my vet too when I see her.