The result of '-2^4' in algebraic notation is utterly dependent on the precedence and associativity of the system evaluating it. Without all the parenthesis needed to completely disambiguate the expression the result can be either -16 or +16. Neither result is right nor wrong.
Most...
Just a reminder about this comparison (one of the most level and impartial that I have ever seen):
http://www.laurowski.de/hp49/ti89hp49_comparison/ti89-hp49.htm