Why is magnesium L-threonate dosage less than other types?

You should not be getting all of your RDA for magnesium in a supplement. It's meant to supplement your diet, not replace it. Threonic acid is also more bulky than other magnesium counterions, so the threonate takes up more weight. If you are just looking to get the most magnesium into your body as you can, Micromag is the best one out there. It's the highest % of elemental magnesium per mg, and has very good bioavailability. You use magnesium L-threonate for the brain effects, not to replace your entire magnesium intake per day.