Probably they are 300/300 with just the items they want in their inventory. For example, if they are only drilling for the Poppuri Boxes, then they would have one Pop.Box at least in their inventory then fill it with tons of stuff (stones, pots, etc...) so they would reach the limit of 300/300 and so they will only catch what they want (which in this case would be the Pop.Box).
Don't know if I explained well or if it was this what you were talking about