Different brands of tires in the same size can have a 1-2 pound difference depending on internal number of ply and strength of materials.
Nylon, polyester, rayon, kelvar, natural vs synthetic rubber, steel vs aramid
Michelin is usually the lighest for strength of construction, something they pride themselves on, as they can spend a few dollars more on exotic strong materials, which increases durability substanially.
http://www.rubberdivision.org/...3.htm