Speculation here, but it could be the upmixer that is being used. Many movies and such are 5.1, so if using something Like Dolby DTS Neural migght add in those rears.
Plugging into front height could be a problem... if they are not assignable to play the Rear Surround Signal.
Similar to above, the Height Channels are not usually encoded into 5.1 mixes... and this ends up getting 'created' in something like Neural.