I had made a High Sierra clear installation in my old Mac Book Pro 13"(Mid 2010) where I had put also a new SSD HD. After this I restored my Yosemite Time Machine backup. It said that it will erase my HD and I clicked YES and continued to restore. It had restored very well all my files but there is no High Sierra there anymore. If you go to "About this Mac" you see the Yosemite 10.10.5 which was before. The question is: Could my Yosemite Time Machine backup that I had, restore the Yosemite operating system and wipe out High Sierra? I needed to have the High Sierra as operating system and my older files which I thought Time machine would restore them.
Yes, this is different from iOS,iOS only backup data and you can't restore your backup to a low version.
Time Machine will backup all you files include macOS itself.
Recovery from Time Machine will restore the entire partition.
So this is normal and you don't have to worry.
You can download High Sierra then upgrade instead of clear installation.
I have upgraded many times without problems,from 10.10 to mojave.