Chess matches usually involve multiple games, so you don't need to win every game to win the match. It's not uncommon to play trying to avoid a loss (going for a draw), instead of playing riskier moves in an attempt to force a win.

Because they may be content with drawing as black and seek an (ever so slightly) easier win as white. It depends on many factors: relative player strength, duration of the match, how players are paired in tournament rounds, current standings in the tournament, and so on. A computer won't do this kind of reasoning.