Why would that be the case?
Haven't come across this one before .. a quick look through the -20 TCDS doesn't help me much ....
A stab in the dark would be to query whether the particular engine (or installation) has an operational limit below that temperature which would restrict thrust output a little .. ie a constraint on missed approach performance limited RLW ? The aircraft TCDS appears to be constraining the original 11000lb instead of the higher weight ...
No doubt, someone who is familiar with the AFM will come to the rescue shortly ?
Now you have me interested .. if there are no answers it might be a case of checking with Brisbane to see if the OEM can throw some light onto it ?