A cold shouldn't cause a bunch of WBCs in the urine. That doesn't make much sense to me. The urine may have been high with another test like "leukocyte esterase", which is produced by WBCs. This can be high if the urine sample isn't a "clean" sample and happened to pick up other cells from the surrounding tissue. But WBCs in the urine is generally pretty specific for infection.

Did they give antibiotics before collecting the urine? That could have influenced the culture. Otherwise, you may never get a satisfying answer to your question.