Funny enough, I learnt about this from Noel's Retro Lab YT channel last week [0]. When I saw the title, I figured it is about CPC 472 before loading the page. Yup it is. I recommend a video to watch, it is fascinating of why they did and Noel shows the breakdown of this unusable RAM in the video.

[0] https://www.youtube.com/watch?v=_ErfWfjN9iU