It's also often also part of the employee retention plan. If you discover your boss has lied to you shortly after an IPO or a merger, then odds are good they were trying to hold onto you by any means necessary until their bonus was paid out.

I've had several people I trusted destroy our relationship this way over the years.

The s1 lays out the exact schedule and let me tell you it is not a normal schedule. Is that a surprise anymore though?