rdr,
on most flights, i have noticed skippers keeping the loadsheet to themselves, as if to protect the crown jewels. rarely do they xcheck the numbers on a manual loadsheet, and even rarer, is there an attempt to concur or physically show the f/o, the numbers in question. mostly he will blurt out the numbers, followed by the f/o calculating the required data. the pressure on making schedule plays a part too as invariably, the loadsheet appears a minute or two before departure time. lmc's notwithstanding, there is a general rush to close the doors and get going.
unless there is a strong SOP, a mistake can be made at the 3 areas of required reference, namely the loadsheet, rtow charts/opt (on board performance tool), or FMC insertion.
and the unfortunate recipent of any penalty is quite correctly, the professional who signs above the dotted line. the buck has got to start, or stop somewhere.
True, the loadsheet appears just minutes before pushback, but the numbers are already briefed and shown to the operating crew one hour, maybe more, before they enter the aircraft, as all of this are also in their FPL. Also, it's true the PIC signed all the required docs, yet copies are also provided for all other crew.