BC and the real DAT are not scored point for point. They are percentiles. The mean is around 17.5 with a normal distribution bell curve.
For example a QR score of 18 has a range of about 8-10 questions. But once you get up into the range of 21-22+, 1 question may or may not change your score. Even on the real DAT, sometimes you can miss ZERO questions but you won't get a 30 because that isn't how the bell curve is distributed. Its best to not get too caught up with the numbers. Focus on your raw score rather than the adjusted percentile.
It sounds like you are doing pretty well with things if you are only missing 5 questions. I wouldn't worry too much whether it translates to 20 or 22 or 25. BC is meant to be a tool to help you learn the material that you missed. Just keep working to perfect your minor mistakes and you will do great on the real thing. 👍