MEAN WELL specification shows a higher working temperature than MEAN WELL’s label and safety report. What is the reason for this?

MEAN WELL’s specification shows the absolute values which were verified during design quality verification tests. Those condition are guaranteed by manufacture from quality and warranty perspectives.

When certifying a power supply according to a certain norm, there is normally a requirement described in this standard a certain tolerance which must be considered. (See also FAQ: Why Is The Input Voltage On The Label Different From The Input Voltage In The Spec? For Example, The Specification Shows Is 88~264 VAC While The Label On The Power Supply Says That It Is 100~240VAC?) 

The specification shows what is possible, the report and label of the power supply shows what is approved by the certifying body according to the standards.

Besides the difference due to tolerance there might also be another reason why the specification and label/test report show a different temperature. For example, if the power supply needs to be derated at a low voltage input such as 100VAC, the label and test report might show the max temperature at full load based on this low input.  

Different standards might have different tolerance requirements and different ranges this could mean that the most conservative value, or multiple values will show up on the label of the power supply.