This is a mirror issue of the Argument element ordering.

Environment variable order may be significant if the values are not constrained to be literal. 

The order the Envrionment elements are concatenated in is not specified. There seems to be an assumption they are done 
in 
the order parsed in the document. AFAIK there is no assurance that element ordering will be preservered.

A short sentence requiring implementors to maintain envrionment ordering could be added to eliminate confusion.