Help with tar -u

[Lootar's Tales]

Hey guys!

Today with some extra time, decided to improve a personal backup...

 Scenario:

~/Downloads/apk/:

total 29928
-rw-r--r--.  1 11907983 abr  8 22:58 'Call Blocker Block Callers_v5.0.3_apkpure.com.xapk'
-rw-r--r--.  1  5247 mar 23 09:58  firewall.user
-rw-r--r--.  1 18709780 mar 27 11:18 'SIA Obsolete Version_v2019.06.11014_apkpure.com.apk'

tar -cvf ~/backup/backup.tar ~/Downloads/apk/*
All is fine.
-rw-rw-r--. 1 30627840 abr 15 19:57 backup.tar

tar -uvf ~/backup/backup.tar ~/Downloads/apk/* (-u, --update only append files newer than copy in archive)

-rw-rw-r--. 1 61255680 abr 15 20:04 backup.tar

Again:
-rw-rw-r--. 1 91883520 abr 15 20:10 backup.tar

Is t suppose it just to add new or modified files? In this case, nothing has changed.

And now?

Many Thanks!