I am regretfully without a reliable internet connection at home so haven't been able to get in to sort out my PI post Incursion updates but with a bit of Googling around I found this updated PI guide on Keith Neilson's EVE site I am Keith Neilson.

He mentions the numbers you have asked about in the passage below copy/pasted direct from the page.

"This is where you select how many Extractor Heads you want to use. Each one you activate requires power and CPU resources. When active they will appear on the planet’s surface and can be freely moved until the extraction program is installed and submitted. The numbers next to each active extractor head give you a rough idea of how rich the vein of resource that they are directly over is and will change as you move them around. They will also indicate if any of your extraction zones overlap with a red minus figure under the main number."

The whole article seems to make sense to me even without viewing the screens directly so maybe you can work out what you need to know from this.

If this question is still unanswered by the time I finally get back online properly I will for sure be making my own investigations and will report back then.