One counts only what bacteria can actually digest in five days. The other chemically torches almost everything oxidizable in a few hours. They are not the same number, and the gap between them is where the real engineering information lives.
Both tests exist to answer the same underlying question — how much dissolved oxygen will this water "steal" from a receiving stream, or how hard will a treatment plant have to work to stabilize it? But they answer it in fundamentally different ways. Biochemical Oxygen Demand (BOD) measures the oxygen that living microorganisms actually consume while biologically breaking down organic matter in a sample over a specified time — the standard test, BOD5, incubates the sample for 5 days at 20°C and measures the dissolved oxygen a real bacterial population used up. Chemical Oxygen Demand (COD) instead measures the total oxygen-equivalent required to chemically oxidize the organic (and some inorganic) matter in a sample, using a strong oxidizing agent — typically potassium dichromate under acidic reflux — in a lab test that takes a few hours rather than days.