Why, if god limited man's life to 120 years, did people live longer?

I apoligize if this has allready been said,but in Genesis 6:3 (all quoes in KJV) "And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years. "The creation of Noah, along with his wife was mentioned before this. Sarah was not mentioned, but this does not matter because she married Noah, and the names are used for reference. Also, the statement quoted previously could be referimg to any period of time, as it is not mentioned when god said the statement.Furthermore, in congruence to the Flood Theory, Water evaporated in the sky ( as evident from Gen 1:7 "And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament: and it was so. ") This could be cause to slow the penetration of UV rays, and thus extend the lifetime of the occupants of the planet earth. The quote Gen 6:3 could be referencing after the flood, when the earth would be in its final resting state (without much of the oceans in the sky)