simply put: they fooling you, or are clueless themselves. All pool balls of a given set weigh the same, set being 1-15 + cue. although the pool balls today are currently made out of different materials( price being the major factor)
to sum it up: all the balls weigh the same, but different materials play a factor, and diff materials should never be mixed
The specifications for pool balls state that they should weigh between 5.5 and 6 ozs, or 156 to 170 g (see link, section 16). This does provide a range, but certain number balls are not purposely made heavier or lighter than the other numbered balls. Higher quality ball sets will be made to stricter tolerances though.
Some bar box tables will have heavier or lighter cue balls though so the table can distinguish it from the other balls.
to sum it up: all the balls weigh the same, but different materials play a factor, and diff materials should never be mixed
Some bar box tables will have heavier or lighter cue balls though so the table can distinguish it from the other balls.