On many types the difference is down to how the pax load is trimmed. On all manual loadsheets it is by area or bay split. Computerised loadsheets vary, some trim by area and others by row. Trim by area should give the same as your manual loadsheet but trim by row could give a very different result as it takes account of who sits in which row, if you are bored try moving a few and see what difference it makes! One good example that springs to mind are Swiss Er145's, they trim by Class. if its out of trim and not full you can change the version to a better business/economy split and get the aircraft back in trim without moving any pax. The difference between manual and computerised doesn't actually matter as both types are legal and still leave room for error as they use standard pax weights and often standard bag weights. If the limits shown on the loadsheet were the physical limits of the aircraft you would have to weigh all pax and plot every row, because of this most Captains won't have a problem with your 'thick' pen.