They made the battery on the 13" 5% smaller than the previous generation. They also write that they tested the device with auto-brightness disabled and brightness set at 50%. Not sure who the brightness slider works on the new iPads since the iPhones don't get max brightness unless auto-brightness is enabled. So 50% might be 1000/2=500 nits on the M4 iPad Pro and 600/2=300 nits on the M2 iPad Pro, or they might both be about 300 nits.