When a polygon intersects more than one pixel, and each pixel within the polygon occupies a different area, does the mean take into account the area of each pixel and its correspondent value (i.e. weighted average)?

When I used Zonal statistics to determine the sum of precipitation in each sub-basin, the sum of precipitation for those sub-basins smaller than a pixel was lower than the value of each pixel that intersected the polygon (e.g. the sum of precipitation for the polygon was 50 while the value of precipitation of the pixels that intersected it was 300 and 150).

Does it consider the area of the pixel in the polygon to calculate the sum? (i.e. if only half of the pixel intersects the polygon, only half of its value is summed?)