Anyone know why is
while Usb disk of capacity 16 GB is fresh formatted and mounted ?

If you convert GB to Gib your actual disk is 14.2
Actual space = total - metadata - reserved space
13.3 = 14.2 - ~2% total - 5% total

got 10%

= ~ 14.9 - 2% -8%
= ~ 14.9 (1-10%)

all in GiB

it's ~ , apprx. as its reserved will adaptive to change due to file system block size

extN filesystems reserve space only on system partitions, there should be no reserved space on external disks. Anyhow, it can be checked/changed with tune2fs.

OP: exaggerating the size of flash drives is not uncommon, actual size can be checked with e.g. lsblk -b /dev/sdb

Your correct it should be 14.9 instead of 14.2 and I believe metadata for ext4 is more like ~3%. The numbers don't quite match up.

Most Linux filesystems allocate at least one "whole block" to any file – "reiserfs" being a notable exception although its author ran afoul of the Russian mafia and is now out of the picture. (Seriously ...)

A certain amount of space is needed by space-allocation data structures and by directories. USB thumb drives carve off a tiny amount of space for who-knows-what.

You should also make sure that the filesystem is of a proper size to actually use all of the available space. (For example, when you expand a "logical volume" using LVM, you must then resize the filesystem upward to begin to actually use it – a process that does not require downtime.)