The fielding and range values are a just a quick reference guide to represent a player's ability. His actual fielding percentage and range factor* can be found in his actual fielding stats on his player profile. Although two players may have the same defensive rating (e.g. 'A+'), one may be slightly better than the other. For example, if the cutoff for an A+ outfielder was a .990 fielding percentage, then you could have an outfielder with a .990 and another with a .999 and both would be labeled A+.
Players who have played at a position but less than 20 real-life games are penalized. This is incorporated in the ratings and the statistical inputs. Players who have not played at a position will incur a greater penalty than this, based on ratings and inputs at the primary position as well as which way he moves on the defensive spectrum (SS - 2B - CF - 3B - LF - RF - 1B). Moving left is a stronger penalty than moving right. Playing players at position which they never played that season is not recommended.
*Range factors are based on WhatIfSports.com's Relative Range Factor values that take actual stats and better account for actual team makeup and balls in play allowed.