I'm not B737 rated any more, but here are a few
GUESSES which might inspire further research -
Guess #1 : What is the lowest temperature for the environmental envelope of the aircraft? It would surprise me if colder than ISA-16 was not acceptable at lower levels, but most environmental envelopes "go vertical" at higher (e.g. cruise) levels, where a lower limit of ISA-16 sounds feasible.
Guess #2 : It could be a Hexidecimal / Octal problem in the FMC programme, and 16 is the magic number (although F=15, we are talking negative numbers). Hexidecimal / Octal limitations do show up annoyingly in other areas of FMC performance, e.g. for the (Boeing) FMC that I use, the greatest arc that may be drawn is 511 nm radius, check that with Octal and the &H function of your calculator!
Just a couple of guesses to inspire research, standing by for responses from REAL B737 people!
Regards,
Old Smokey