Skewing your elemental resistance in favor of one element will increase the likeliness of getting shards of that element while gathering. If you're in a region already favoring the element you're raising resistance for, you get nothing but those shards. For example, if you use +wind resistance items/materia while gathering in La Noscea, you only get wind shards. I assume it has something to do with that, haven't seen other effects from elemental resistance on gathering personally.

I sometimes stick weak veil materia on SB off-hand tools if I want to control what shards I get.