the pts mentioned are the maximum pts, if all 3 round robin matches have been won by the player.
That means 125 pts (losing the match) + 125pts (winning the match)
so reaching the final means: RR-pts + 330 pts
Winning the final means: RR-pts +330 pts + 420 pts
max. pts sum winning the WTA Finals is:
3 x 250 pts + 330 pts + 420 pts = 1,500 pts