Lance, there are a number of things that contribute to the final weight. One is, of course, the amount of water retained. However, longer coagulation should lead to increased retention, not less. However, the cut size of the curd makes a difference, and even more the amount of stirring and/or cooking.
Equally if not more significant is the amount of protein and fat in the milk - and this can vary widely. In fact, with any given cow, it naturally varies widely at different times of the year. Store bought milk is "standardized," which in theory means that you won't see the seasonal variations as much ... but I've still seen significant variation across time and brands.
One more factor: my sense (not sustained by any extensive experimenting) is that yields from raw milk may be higher than from store-bought - though, as noted above, much more affected by seasonal variations. My guess is that most if not all writers of books have access to raw milk - which, unfortunately, I do not. I've noticed this especially in the way certain things are supposed to work (e.g., flocculation test), that never work very well for me - except for the one time that I did get my hands on some raw milk. That was an "aha" moment that made me feel much more confident about my other results; I wasn't doing it wrong, I just was doing the best possible with the P&H milk available.
All that to say, I wouldn't be concerned. IMO, the main thing at this point is whether you get a cheese you like. Secondary is whether it approaches the taste and texture of what you were aiming for. A long, long, long way third - actually, somewhere near the bottom of the list - is whether you got a certain yield.
