Why did my S6 Fmodel update itself?

[Android Central Question]

I bought it used and it was running android nougat. It kept promting me to update the phone, a 500mb+ update whenever i restarted my phone. I kept postponing it. Yesterday night i slept and when i woke up the next day the software update notification that always remained in the notifications bar says"The device has been updated" Why did it auto-update when i had turned it off. The software now says 7.0 and the clock widget's font looks updated. Why did it do so?
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[PapaGary]

Android Nougat is 7.0.

 

[Rukbat]

I don't know if it's just Samsung or everyone (it happened on my Note 3 also) but you get about 10 or 15 times to postpone it, then it just installs anyway. To freeze your phone at the ROM it's on, you have to root, then disable the update app.

(A 500MB update is just a fix and security update, not a new version of Android.)