Why is Magnesium Oxide so bad?

I keep hearing that magnesium oxide is not a good supplement to try for magnesium because it's said your body only absorbs 4% of it. But how could that be possible when the National Institute of Health says that magnesium oxide has the most magnesium content than all the other popular magnesium's ( magnesium citrate and chloride) combined. Where exactly is the source where they claim it only absorbs 4% in your body does anyone know where that information came from?