2.1.5 [W3C-XSD] Section 126.96.36.199, Order relation on duration
The specification states:
Implementations are free to optimize the computation of the ordering relationship. For example, the following table can be used to compare durations of a small number of months against days.
Some comparisons that are indeterminate are considered to be
true, such as the comparison between 1 month and 30 days.