The Mei Hashiloach says another idea elsewhere that relates to this one.
The pasuk says "לעולם ה' דברך ניצב בשמים" - "Forever Hashem your word stands in the heavens". Meaning, that Divine decrees are malleable and can be changed to mean something else. The words are there and subject to reinterpretation.
So just like we can shift our perspective and see a seemingly negative situation in a positive light, Hashem's decrees can also be shifted.
So when it was decreed "ועבדום ועינו אותם ארבע מאות שנה" - the servitude will be for 400 years, that was ultimately taken to mean 210 years [either because it was so difficult or it started earlier or for other reasons].
#lifeisfluid
