Why do identical folders on two different hard drives of the same file count and file sizes, take up a different size on disk?

Question

Both have

• 216,540 files
• 52,103 folders
• Size: 278 GB (299,363,715,785 bytes)

But for one hard drive the size on disk is 278 GB (299,672,182,784 bytes) and for another it's 278 GB (299,672,211,456 bytes) bytes).

That's a difference of 28,672 bytes or 28.67KB.

Why?