Option A: Series B investor buys shares from the Company. Company issues new shares and the Series B guy owns 20% of the company. This dilutes everyone else's ownership (for him to own 20%, everyone else's stake has to come down).
Option B: He buys all of his shares from existing shareholders. This requires no additional shares so people who are NOT cashing aren't diluted at all.
I'm just guessing here-- I've never taken money off the table. But I can't imagine it working any other way.