As above: Just forged some Dragonplate armor. It got an armor rating of 200.
When I tried to enchant it (e.g. with Fortify Health) - the armor rating would drop to 155 or something.
So, it's either AR 200 but no enchantments, or lower AR with anchantments.

Why is this so?

I have never noticed this for other armors?

You might be looking at it the wrong way. Do you have a perk that will give you a bonus when wearing four armor pieces of the same type?

If so then you have to wear the item to get the bonus. If you are not wearing it then the bonus from the perk will not get applied and all armor ratings will drop.

Actually, it's a glitch. The perk mention above causes it.

When you have the glitch then go to enchant a piece of armor you are wearing that has been upgraded via smithing then the upgrades you made will be removed because the enchanting table will give you an enchanted piece of that armor but with the original stats instead of the upgraded stats.

To avoid this glitch, take off all armor before enchanting it.