I suspect that what is going on is a similar process to the old Commodore diskette system, which I guess you will be familiar with.
When deleting a file from a diskette, the file was not physically deleted, but the "pointers" to it were set to zero, instead of pointed to the track and block where the data resided.
Those of us who had the right software could undelete files if necessary, although they could be corrupted if overwritten by new files.
Unlike a PC, where a program can consist of hundreds of file, registry entries etc. the apps on an iPad seem to consist of one file plus an icon, so there is not much to delete. Zap the icon, and reset the pointers and the space is free.
Much of this is speculation, but it seems logical to me.
Sent from my iPhone using iPF