Selected Temperature Derate (B744 FMC)

[QAVION]

Hi, folks.
In various airline 747-400 ops manuals, it is written that the range of permissible entries on the THR LIMIT page for SELected temperature (for engine derate) is 0degC to 99degC. However, if, for example, I enter 99degrees, the value which appears on the SEL temp line is not 99, but something substantially lower (e.g. 59deg). There seems to be an imposed limit... but, from one aircraft to another and from day to day, this limit appears to change.

I'm wondering if the value which appears is dependent on TAT (or OAT) (e.g. TAT + fixed value(?) = displayed limit).

Any ideas?

Thanks.
Rgds.
Q.