Official comment
That's normal, and it has nothing to do with rekordbox, it's simply how the drive manufacturer has marketed the drive size and math.
32GB is not 32,000,000,000 bytes... a KB is 1024 bytes. 1MB is 1024KB. 1GB is 1024MB. So effectively, 32GB is 34,359,738,368 bytes.
"But!" you say "Why is my drive not reporting as being 34GB then?" Well, that's thanks to marketing doing the math backwards for you. So the computer divides 32,000,000,000 by 1024 = 31,250,000, then by 1024 again = 30,517, then by 1024 again to give you 29.8GB.
And then sometimes the drive isn't exactly that big because of marked bad sectors, a partition (whether hidden or visible), etc.