Because two years ago Twitter raised at $8 bil valuation http://allthingsd.com/20110801/twitter-confirms-funding-with... and private market transactions have valued them in the neighborhood of $10 bil, which might have impacted the FMV for new hires.

If you're joining the company today, there better be significant growth story to justify 4x-5x growth from $10 bil baseline. If it's just 2x-3x, heck, joining GOOG, YHOO or AAPL might yield the same results if you timed it right.