Why did od repeat exam after ophtha md did one three weeks ago?

Eye exam. The simple reason is because that is just what optometrists do. Strictly speaking it is not necessary, but it doesn't do any harm either. Lots of time an od will re-check the eyes after an ophthalmologists exam because they will want to see something the ophthalmologist may have observed that they didn't see previously.
Repeat Exam. Sometimes exam are repeated to uncover a discrepancy. Sometimes to confirm a diagnosis. Sometimes they are trying to bill for this extra service. Best thing to do is ask the doctor why he is repeating something. I personally don't rely on an exam from another doctor and always repeat for completeness and accuracy. .