I'm trying to improve my understanding of pitching rates, and I'm struggling with some apparent inconsistencies.
An often used starting rule is to pitch about 750,000 cells per milliliter per degree Plato for ales, and twice that much for lagers. So for an example batch of 5 gallons (20 liters) of wort at 10°P (OG = 1.040) you'd need about 150 billion cells for an ale and about 300 billion for a lager, in order to get a consistent and reliable fermentation.
Common wisdom also has it that you usually don't need to make a yeast starter when pitching dry yeast into a medium-gravity wort. Practice bears this out: many home brewers (including your's truly) simply chuck one packet of Fermentis or Lallemand/Danstar dry yeast into a 5 or 6 gallon batch. Most of the time we don't even bother pre-hydrating dry yeast if the OG is less than 1.050 or so. Let alone making a yeast starter, because in practice you simply don't need to. Just add one packet of dry yeast to 5-6 gallons of wort, and you'll be fine. And indeed this has always worked for me.
Common wisdom also states that manufacturers of dried yeast pack tons of cells in their products so that after two years of storage there will still be enough viable yeast cells left to do the job.
Looking at US-05 as an example (although the figures work out to pretty much the same for all the other popular dried yeasts), Fermentis specifies that it contains at least 6 billion cells per gram of dried yeast when packed, which means that for a standard 11.5 gram packet of US-05 we're looking at about 69 billion cells per packet.
Fermentis' datasheet recommends pitching 50-80 grams of US-05 into a hectoliter. At the specified 6 billion cells per gram, this works out to not pitching the recommended 150 billion cells in the above example batch of wort, but only 60-96 billion cells.
The instructions on the packet are even worse: there it says that one 11.5 gr. packet can be used for 20-30 liters (5-7.5 gal) which works out to a pitching rate as low as 230,000 cells / ml / P rather than the recommended 750,000.
And that's even ignoring those common, real world cases where the OG is higher than 1.040 and/or when the yeast approaches its 'best before' date. From what I understand the loss in viability of properly stored dried yeast over two years should not exceed 10% or so, yet it is still a factor.
So. On the one hand, common wisdom and real world experience tells us that one packet of dried yeast into a 5 gallon batch of wort up to an OG of 1.050 or so works fine. On the other hand, doing the math and checking that against the popular "750.000 cells / ml. / °P" rule states that this shouldn't be enough by half at best.
What's going on here?