Why du command is showing 501MiB?

Question

I'm making a 500MiB binary file like this:

$: dd if=/dev/zero of=mlinux.img bs=1048576 count=500

Then when I do:

$: du -b mlinux.img

It returns exactly 524288000

But when I do:

$: du -BMiB mlinux.img

It returns 501MiB

Why not exactly 500MiB ?