A good way to avoid parallax is to place a mirror by the scale behind the ball and, by moving your eye, attempt to get the ball and its image coincident when making the reading. Similarly most materials become less bouncy as they get warmer. Temperature range . Croquet is played throughout the year and all over the globe. The temperature range for testing balls is not stated.
Quick calculations indicate that the temperature effect on the dimensions of the ball is minimal.
It is the effect on the coefficient of restitution which is probably the most significant. These are normally a pair metal plates with an accurately machined hole in them. One hole is the minimum permitted diameter and the other the maximum. If there is less metal around the hole it is likely that the metal will have deformed when cut and the hole ends up non-circular. The roundness is a measure of the deviation from being a perfect sphere.