The resolution is defined as 1/3 degree, that means three cells are one degree high or wide. Near the aequatorequator, theythe cells are squaredsquare, but next to the poles they get to long rectangles, ending in a triangle at the pole itself.
If you want a resolution in real meters on the ground, you have to reproject the data to a projected CRS. Then you will get the rectangular size ratio you expected.