Taking Brix Measurements for Our Auxerrois

Recently, my husband was on his tractor and spraying. This is the best time to assess the overall health of the vines and the grapes. While he was in the Auxerrois row, he saw that the grapes were changing color from green to yellow green. This is a sign of ripening or véraison. Last year, with the few Auxerrois grapes that we had, I did take more data points on the progress of the ripening which began on August 16th with a Brix reading of 9. It took two weeks for the Brix to increase from 9 to 13, by August 31. From August 31 to September 15th, the Brix stayed constant, reading 15. There seemed to be a plateau that the sugar level had reached. We harvested the Auxerrois on September 21, when the Brix reading was at 17.
We have a few more grapes this year so we can take more readings with a bit more accuracy. But, as of August 19, our Auxerrois Brix are reading 13.
This is highly unexpected, given that budbreak for the Auxerrois this year was on May 7. Our data point from last year seems to suggests that harvest date is approximately 144 days post budbreak, which when applied to this year would be September 28. One thing is true, it's time to start netting!