Microsoft has quietly been upgrading the CPU on new Surface 2 Pro devices, including replacements being sent out. The old Intel Core i5-4200U has been replaced with the Core i5-4300U, bring the speed to 1.9 GHz (2.5 GHz Turbo Boost) from 1.6 GHz (1.9 GHz Turbo Boost). This should give the newer units a bit of a performance increase.
In a statement to The Verge, Microsoft has confirmed the change:
A Microsoft spokesperson confirmed the change in a statement to The Verge. "Microsoft routinely makes small changes to internal components over the lifetime of a product, based on numerous factors including supply chain partnerships, availability, and value for our customers," says a Microsoft spokesperson. "With any change to hardware or software, we work to ensure that the product experience remains excellent."