Just an observation on my part, by no means backed up by data (just what it seems is happening... I don't count), but it seems that when I search for a specific herb, the reduced time searching actually makes the overall yield for that kind of herb lower than if I were to search for all herbs and throw out everything I don't want.Using your 'herbalism' skill, along with your 'awareness', you can search the area you are in for either any herb, or a specific herb, slightly raising your chances of finding the specific herb you seek, but reducing the time it takes to search the area as you overlook many herbs you do not specifically seek.
This doesn't sound right, of course, but it's something that seems to be true since I've had very high skills in both herbalism and awareness, as there is such a long overall search time and the likelihood of me finding anything at each tick is generally good. If I want a specific herb, I just gather everything and toss out what I don't need.
I don't want to know the details of how the system works (that would ruin the fun), but I would like to know if my observation is rooted in truth, because if it is true, then it is a problem.