In Genesis 6:3, God prescribes a 120 year lifespan just prior to the Flood. Psalm 90:10 does not say the lifespan is 120. It's a poetical reference to us living 70 years, 80 if we are strong. (According to the NIV notes, Hebrew poetic convention called for 80 to follow 70 in parallel construction). Genesis 6 could be setting an upper limit, or given the context, it could be just one way of saying that man is mortal. Psalm 90 is an observation fitted into a poetical account of our fleeting existence.