Works All Day
I have 2 Ions. Both in men's size Large.
One is the original Pertex version. It weighs 3.155 oz. on my digital scales. It breathes very, very well. I wear it over a merino wool or poly shirt typically. It has been great over the years. I leave it in my pack as one of the 10 essentials. I've used it in the Andes, Rockies and Blue Ridge while hiking, cycling, sailing and paddling.
The other Ion is newer, 2006. I had a Pertex tag, but I think they started using a different DWR because it doesn't breathe as well as the original. It does resist water for longer. It weighs 3.327 oz. on my scales.
You weight weenies need to get a life. Variations in the DWR coating, fabric and what not, though within manufacturing tolerances, can explain the weight difference. If you want it to weigh less, cut off the metal zipper tabs and use cord instead. Dental floss would be lighter.