Charles Ellicott Commentary Numbers 18:30

"Therefore thou shalt say unto them, When ye heave the best thereof from it, then it shall be reckoned unto the Levites as the increase of the threshing-floor, and as the increase of the wine-press." — Numbers 18:30 (ASV)

As the increase of the threshing-floor.— As the tithe rendered to the priests was to be regarded in the same light as if it had been the produce of their own labor (Numbers 18:27), so what remained after the heave offerings had been duly set apart was to be reckoned as much the property of the Levites, and to be treated in the same manner, as the corn of the threshing floor and the wine of the winepress of the rest of the Israelites.