The syrup comes from the motion of the fluid in the tree - when it gets cold at night the sap goes into the roots, when its warm during the day the sap rises. so to get a good flow you need cold nights and warm days.

during the flow the sap picks up the maple taste from other bits of stuff in the tree - grade b is actually the first sap produced in the season and has the most of this 'stuff'. As the season goes on theres less and less of this stuff so then you start making grade a and fancy.