Why is the peak power for Transmit Analysis 3.78 dBm, but with Zero span it is 3.66 dBm?

The Transmit Analysis Output Power result is not 100% consistent with the test in the SIG spec. It is measured in the IQ domain, not in power domain, so there is no VBW function. The most important factor is the bucket width. In zero span, the bucket width = 3 ms/1001 = 3 us. In Transmit Analysis, bucket width = 1/15 MHz = 0.066667 ms.