Makes absolutely no sense at all. Lamellar armor has a higher degree of movement than plate armor. If anything, armor should REDUCE effective agility by a much higher degree
Someone with 3agi should actually turn their character slower, limit weapon release rotation degrees MORE, poorer weapon arcs, slower draw/sheath weapon times, slower feints and block recovery times, etc
Yes, it has a higher degree of movement than plate, but it also requires a higher degree of proficiency with making blows land in the correct locations. If you don't get hit on that vest, all you've got is a padded shirt. In plate, whether you get hit in the arm, the chest, the leg, whatever, it's plate, so you don't need to worry nearly as much about trying to maneuver around the blow. Hence why agility would have the penalty to armor, while strength has the penalty to movement.
You need to be able to move well in order to fully utilize partial armor like those I mentioned. In plate, you don't need to be able to move nearly as well to make full use of it.
The movement penalty on agility I mentioned was something I was thinking to be very small (like .2% or less per lacking point of agility) and was mostly there for combat situations on the realism basis that if you don't have the agility to properly use your equipment, you'd have to spend more effort on moving defensively, and would have less concentration to put into footwork. Anyway, just scratch that part if you don't like it.
The main point of the suggestion was to promote more balanced builds, and to give penalties to extreme builds (particularly extremely high strength). As odd as it may sound with me being in KUTT, I'm not fond of full strength build characters. Discussions of skill and whatnot completely aside, it's just a hell of a lot more fun to fight someone who has a balanced build.