Most prob didn't realize Imperial is base 10.
The acre is a unit of land area used in the British imperial and the United States customary systems. It is traditionally defined as the area of one chain by one furlong (66 by 660 feet), which is exactly equal to 10 square chains, 1⁄640 of a square mile, 4,840 square yards, or 43,560 square feet.
A chain is a larger unit of length measuring 100 links, or 4 rods.