[Q]What's the difference for /system under Recovery mode and Root privilege topic

I want move my app to /system/app/, I did follow steps under ROOT privilege

Code:

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mount -o remount,rw /system /system
cat /data/local/tmp/abc.apk > /system/app/abc.apk
chmod 644 /system/app/abc.apk
mount -o remount,ro /system /system
reboot

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but the abc.apk file disappear from /system/app after reboot.

if I use update.zip package under Recovery mode to write the abc.apk to /system/app, it will be normal after reboot.

so, what's the difference for wrinting system partition under Recovery mode and ROOT privilege? :confused:

Thank you very much!