Why the input voltage marked on the spec sheet is 90~264 VAC while the label on the power supply says that it is 100~240VAC?

                                                                                                                
During safety verification process, the agency will use a stricter standard (IEC60950 uses +6%, -10%) of the input voltage range labeled on the power supply to conduct the test. So, operating at the wider input voltage range as specified on the spec. sheet should be fine.
The narrower range of input voltage labeled on the power supply is to fulfill the test standard of safety regulation and make sure that users insert input voltage correctly.