My apple watch's memory is 63gb not 64

Those of us that grew up in the early days of the computer industry, think megabytes, gigabytes, terabytes, petabytes are powers of 2. Or an easier way to think about it multiples of 1024.

Somewhere around the time disk manufacturers started going from megabytes sized disks to gigabyte sized disks and the IBM PC & clones were getting big, the disk manufacturers switched to multiples of 1,000.

Their argument being mere mortals did not understand powers of 2. And most likely they got to advertise larger storage capacity with the swipe of a pen, and no engineering work.

Conversely, RAM sizes have stayed power of 2 based.

You don't. the difference is due to the difference in the way "GB" is define technically, and for marketing.

Don't stress over it.