You're always going to get some spillover, but generally over the long run you end up with more of what you're aiming for. I know when I'm fishing for a particular fish, it always feels like I'm getting nothing but everything else, but after 4 hours of fishing I always have way more of what I was aiming for to begin with.

It doesn't take 4 hours of gathering to see the difference, but overall it's a "long run" kind of thing.

Then again all I do is fish, so there is a chance botany is bugged. Just adding my observations.