It is because cell a undergone anaerobic respiration, that occurs in the absence of oxygen, which produced lactic acid especially in human muscles, and because it's a incomplete oxidation of sugar, therfore the atp, which is a kind of store energy, is less.
However cell b undergone aerobic respiration, which produces water molecules. It occurs with the presence of oxygen, and its a complete oxidation of sugar, therfore it produces more atp.