Pre surgery PSA should be flushed from the system within 6 weeks after surgery, which usually results in very low numbers by then.
Lab errors / one-off unexplained elevated readings do happen, and there are reasons why surgery is not always successful as a cure.
Without knowing your full details, it sounds as if there was no indication of potential problems of this kind with your original diagnosis or surgical reports which makes a PSA this high very unusual.
To use the analogy by Dr Myers in his book, prostate cancer treatment can be like golf and we need to play it as it lies: obviously you need the early repeat test to more accurately gauge what you're dealing with.