A 1" T fits the pipe and a 1" valve does not, why?
That valve is for a different type of piping system called CPVC and the sizes and fittings are different. The difference is deliberate because CPVC is rated for hot water whereas regular PVC is not; they don't want people to accidentally (or deliberately, to save money) install regular PVC pipe or fittings on a hot water system.
Look at the valve label.