It is usually the Time of the Season. Early in the season when it's cold, the sap gets boiled with almost no bacterial growth. As a season goes on and it warms a little bacteria can grow in the sap, the bacteria eats the sucrose breaking it down to glucose. Glucose is more susceptible to the malliard reaction and the syrup browns more. I myself prefer the darker syrup as it has a more robust maple flavor. I get just a little bit of light syrup at the beginning of the season and it almost has a vanilla taste to it. I have enough customers, including my mother in law, that only want the light stuff.