Devo-tee....
Every doctor is different with the pre-op diet based on weight, liver size, etc. So that part you are correct. However, even the variations of the doctors can be very different with in the practice itself. I chose my doctor out of the group because his preop diet was Protein and vegtables. So I would go out and order chicken and steamed veggies. The other doctor in the practice was two weeks of liquids. My doctor only made me do the pre op diet for a week before because I am on the thinner side and in almost perfect health other then the weight.
As far as post op. For bypass patients they have to be on liquids for a lot longer because they can not handle food. They have to let all those incisions heal. For banding, we are trying to let the inflammation that happens from placing the band to decrease. You don't want to start mushy food if you are still inflammed, it can cause a lot of pain. So letting the inflammation go down is super important. However, if after a week on liquids, I would recommend talking to your doctor about it. My doctors rule is 2 weeks liquids, 2 weeks mush, 2 weeks solids. If after 1 week of liquids we feel hungry then we can go to mushy. I don't think this completely answers your question but I hope it makes it more understandable at least a little. It really does depend on the doctor. If you are starving on liquids though it is super important to communicate that with your doctor.