Is is a static file? A log file that is being written to? Regardless of the difference between the file system, does it need to accurate down to the byte?
Most likely it is the difference between size of bytes which is what you get in Length but then there is the “Size on Disk” which is the sum of the block size like 2 files that total 2.8K in 2 files but on disk it is 8K because the drive was formatted in 4K blocks.
It works for me. Length agrees with Size:, but not “Size on disk:”. I had to do “get-childitem -force” to get any hidden files. System files don’t seem to give it any trouble.
# find hidden files
ls -r -force | where mode -match h