The GWP value depends on how the gas concentration decays over time in the atmosphere. This is often not precisely known and hence the values should not be considered exact. For this reason when quoting a GWP it is important to give a reference to the calculation. Commonly, a time scale of 100 years is used by regulators.
So it's an educated guess that has lots of flaws just like I said. GHG emissions comparisons between output methods is ridiculous. Because again, water is a GHG emission.
