Then how come plate mail is listed with a higher encumbrance than chain mail in my D&D manual?

Perhaps the same reason that the D&D spells Melf's Minute Meteors and Meteor Swarm have much of their effect in the form of fire damage.

1) Because the D&D designers either didn't know the truth or didn't care and 2) because it works better for game balance.

If you care about this kind of thing I recommend Riddle of Steel.

D&D arms and armor has very little connection to history. Indeed, many historical fighting styles are either impossible or very difficult under the standard rules. (This is true in both 3/3.5 and 4th. I don't know how true it is in earlier editions.) Similarly, arrows are aren't nearly as deadly as they were historically. And then you have ridiculous things like the "dire flail" which seems to be a recipe for getting yourself hurt real fast.