Slurry volume in a starter

I can't reconcile the amount of slurry I get in my 2L starters (made using the prescribed calculators) with how much is supposed to be there based on the following:

Not having a microscope, I have looked through the literature and it seems that there is general agreement (if there ever is such a thing with brewers) that there are about 1.2 to 1.5 billion cells per millimeter of slurry in a typical starter. So I have been crashing my starters and measuring the amount of slurry in the bottom. (I do this by crashing it, decanting and then pouring the rest into a graduated cylinder and crashing again to see how much slurry I have.) So if one is shooting for about 200 billion cells for a 5 gallon batch with a middle-of-the-road gravity and uses the 1.2 billion number, then the required volume of slurry for pitching is 167 ml., or about 3/4 cup (6 oz.). I have never been able to amass this much slurry in a two liter starter. Typically I get about 50 ml.

Do others get that much? Can anybody enlighten me? And yes, the beers come out good but inquiring minds want to know...

 

Oops, I did that #3 thing somehow... Anyway, I step up from slants 3 times on a stir plate, 50ml to 250 ml to 2L. M.Malty's "Proper Yeast Pitching Rates" does state 4.5 billion/ml, but then goes on to say that, according to Fix, in a slurry, only about 25% of the mass is yeast solids. So I'm back to times 4.