iTunes for windows downloaded iOS update using cellular data despite PC internet connection and WiFi being available

I found some pretty stupid hebaviour and wonder why it happened.

I have

-iTunes Version 12.13.4.4

installed on Windows 11

-iPhone 12 with iOS 17.6.1

PC is connected to internet via Ethernet cable connection to router and iPhone also connected to WiFi using same rounter.

What I did:

-Connect iPhone to PC using USB-C cable

-select to trust the PC on iPhone

-Start ITunes

-As soon as iTunes started, there popped up info that iPhone software update (iOS 18.1.1) is available for download

-of the 3 choices I clicked the "download only" button

-I clicked through the license agreements etc, then the download was started in background

I assumed it would use my PC internet connection to download the update package and synchronize it over USB to iPhone for later installation. Or at least donwload over WiFi like it usually does when doing the download on the iPhone.

But no, it appearantly used my cellular data.

As soon as the download progress in ITunes reached 80% of my cellular monthly budget I got SMS from my mobile provider warning that 80% data volume was used up, and only few seconds later that now 100% is used and my internet rate would now be slowed down. Also the download progress slowed down when all data was used up. I cancelled the download.

Checking on the phone what used my mobile data, it looked as it was the "personal hotspot".

I then saw in settings on phone, that personal hot was indeed still activated, so I turned it off and restarted download in iTunes. It then was downloaded in no time.

So, it seems that iTunes preferes using iPhone as hotspot for downloads when available, even for large downloads like whole system updates.

Why it is doing that?

It is pretty stupid, especially considering that previously there were community posts answere that downloading system updates over cellular network is not possible, for example How can I update iOS over cellular data? - Apple Community

Guess, I accidently found a workaround how its possible anyway.

Trying to google this particular issue also found only posts from people trying to download iOS updates over cellular data, which is the exact opposite of my question.