why is the capacity of my ipad 28gb
A certain amount of the overall storage is taken up by the opering system and files that actually make it function (as is the case with any computing device). That is normal.
Some space is used by iOS and the built-in apps, and part of is also down to how a gig is defined : storage capacity reporting.
Also, look to your right under "More Like This" for related discussions. --->
Retrieving data ...