That's normal due to the gradient in the channels and the rate at which you draw off. In fact, at any given moment there is only a small percentage of the syrup that is at the perfect density. The trick is to dial-in the rate of draw-off combined with the feed rate. But you also have varying densities of sap entering the evaporator to skew all your calculations, as well as fluctuations in the barometer. Do you re-check density before bottling?