Apple deep sleep is hit or miss. If the algorithm likes you it will score you high. If you aren’t jiving well with the training data used then it will give you these strange and improbably low deep sleep.
And before people say look at the quantified scientist YouTuber…. I watched it all and Theresa in his video with the Apple Watch displayed terrible deep sleep detection with Apple Watch.
Garmin is much more accurate with Deep Sleep figures than Apple.