What 45 KB is good for
45 KB exists because 50 KB is such a common limit. Targeting slightly below a stated ceiling is the practical way to submit a file: it satisfies the rule while absorbing any difference between how you and the portal measure the same file.
The margin is not superstition. Some systems compare against 50,000 bytes rather than 51,200, and a few add metadata on receipt. A file at 45 KB clears both readings.