New replacement windshields are thinner because of the way they are made these days. In the olden days the ancients sandwiched plastic between two pieces of glass and heat bonded them. Today the next generation "laminated" glass is one piece of glass that is coated. That is why you should never use a razor blade on a new windshield. OEMs started using this manufacturing technique to save weight and money. In all reality though the difference is minimal and nothing some sealer can't overcome.

At least that is what I've heard...