dry yeast has 20 billion cells per gram or 230 billion cells minus I think 4 % per month loss , could be 10% . Wyeast has 100 billion so I suppose that could be the reason . How ever Wyeast does not recommend starters for 5 gallon batches . i.e. The Activator package contains a minimum of 100 billion cells in a yeast slurry.. The Activator is designed to directly inoculate 5 gallons of standard strength ale wort (1.034-1.060 SG) with professional pitching rates. For lagers, we recommend inoculating the wort at warm temperatures (68-70°F/ 20-21°C), waiting for signs of fermentation, and then adjusting to the desired temperature. Alternatively, for pitching into cold conditions (34-58°F/ 1-14°C) or higher gravity wort, we recommend increasing this pitching rate. This can be achieved by pitching additional Activator packages or by making a starter culture. Please see the Pitch Rate section for additional information.
Wyeast recommends 113,562,600,000 cells for a 5 gallon batch of ale . No starter needed .
Now personally I do like to use 2 packs of dry yeast and that makes for some quick fermentation.