You have some good questions. Hopefully I can shine some light on things for you.
First, there has been a lot of additional understanding of yeast cells and pitching rates since the Zainasheff/White book came out. As someone who regularly peruses scientific papers, I can tell you there have been a explosion of papers on yeasts in the past few years.
So, getting to your questions. Yeast companies, such as Wyeast and White Labs, designate their 100 billion cell packages as 'pitchable' for 5 gallons of 12 Plato wort. Their thinking is that the health of the cells in a recently produced package (within a month or two of packaging date) as such that the yeasts can handle multiple rounds of generational growth without losing their ability to ferment and maintain healthy cell walls to survive in a toxic (alcoholic) environment.
Much of the pitching rates comes from commercial brewing, where repitches are common and cell wall health may or may not be examined carefully before tossing in a batch of recently harvested yeast. It aims for a relatively comfortable range of cells which will consume and ferment maltose and other sugars at a quick and predictable rate. With some of the new findings, people on the home brewing level are starting to look at pitching healthy cells in some ample quantity to get a good fermentation. This is present in vitality starters and the SNS (shaken not stirred) yeast starter methods which are now being used by many home brewers.
Next thing to realize is that the 'ideal' pitching rate varies from yeast strain to yeast strain. Some strains do very well and produce desirable qualities when pitched at slightly lower rates (Hefeweizen yeasts and Kveik strains are two that come to mind). Still others perform better and are better suited to completing a fermentation when pitched at levels above the 'ideal' rate (WY1214 Belgian Abbey is one). Then there is the desired yeast flavors which can vary based upon pitching rates which I find predominant in WY1272 American Ale II where a lower rate will produce some nice fruity esters while a higher rate will give me a clean fermentation.
Anytime you make a starter, your actual cell count is really a reasonable guess anyway. Unless, like me, you have the time, energy, and equipment to perform cell counts you really do not know how many cells you are pitching not the health of those cells and the thickness of their cell walls. You can be assured that you are closer to the model generated number than if you did not make a starter but beyond that...
The net effect is that the yeast strains are very much individualistic and sometimes you need to experiment a little to see how they will perform and what they will produce for you. Yeast starters can provide a bit of insurance; though these days unless I am making a starter to split, I much prefer to pitch a new packet of yeast into a small batch and harvest from the trub of that fermentation.
What I can say, is that the yeast companies are certainly the experts and know just what their product limitations are. I can also say that companies such as Wyeast do tend to make sure there are plenty of cells in their packages. I don't think I have ever counted less than 100 billion cells in a smack pack and usually the counts are running 5% to 10% more. I can also tell you that a properly handled package of yeast will have many more cells than the starter programs predict based upon the age of the package. The environment inside the packaging and the nutrition contained within are plenty to keep the cells happy and healthy. Of course, if you leave you pack out in the car under a hot sun for the day, all bets are off.
I think I have answered some/most of your questions. If there is something you are not clear about or want more information, let us know. If I don't know, there are plenty of other knowledgeable people here who do.
