That's likely the reason for the skew. They are trying to retain engineers past the typical time on site.
I met tons of people who did 2 years at amazon and departed to Facebook for a fat offer that was worth far more than the remaining unvested amazon stock.
Trying to nickle and dime people when they come in the door just leads to people departing early. Employee turnover is a massive cost.
At some point it is less about money and more about where you want to spend your time.