Boeing OPT and FMC discrepancies

[Broomstick Flier]

Hi gents,

This one has been keeping me busy reading the 767 manuals, but no luck there. Lets say you make a take-off calculation with Boeing's OPT, ATOW already inserted and equal to load sheet weight, and leave the ATM temperature set as MAX, ensuing result shows an ATM of 58 degrees. When setting this temperature into the FMC or dialling it in through the TMC it automatically limits it to, lets say, 40 degrees.

So why does this happens? I had the impression the OPT software had the same algorithms/performance database as the FMC, and essentially we suply the program with the same data the FMC is able to abtain through many input sources (OAT, QNH, altitude, etc).

Is there any way to predict the max value the FMC will accept?

Thanks in advance!