High Sierra with Yosemite Time machine restore
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.