do you some times just take the answer as it is?

for example, when calculating the margin account balance on future contract. I do not know why you compare each settlement date’s price (winning/loosing) with the previous date’s price, and then accumulate, instead with the contract purchase price. I always thought each day’s winning loosing should be based on the contract purchase price, because you did not really settle the contract, it is just hypothetical. so you always want to compare to the money you spent on purchasing. Apparently, that is not the answer say. So i guess i just need to accept the fact that you always accumulate the loss from previous trading price.