After all, you know how a sandwich tastes after you eat a sixth of it. You usually know if a book is good after you read a sixth of it. The main reason fans aren't supposed to get nervous when their team underperforms in spring training is that reserves and rookies, not starters, get the bulk of the playing time. And even by April they may still have a few kinks to work out. This is especially true of the veterans who play for winning teams and don't generally toil away in winter ball like the hungry first-place Phillies and Twins youngsters.
Baseball players are superstitious.
They believe in jinxes. Turk Wendell eats licorice. The juggernaut Braves choke every year, and the Dodgers are usually inexplicably awful. In the early going, teams like the Cardinals, picked to dominate the National League, often fail to live up to their potential. And teams like the Twins self-actualize because they think they can play and hate being overlooked.