For which models was 2^3>8?

[Andres]

It has been said already, but just in case... The HP=25 needed to allocate ROM space for programmable functions, but the HP-22 and the HP-27 were not programmable, so there was more ROM space available to put more sophisticated code for these math functions. While the HP-21 was not programmable indeed, it has no memory (apart from ACT registers) and I guess its ROM space was also limited. By the time HP-29C, 19C, 91, 67, 97 appeared, more ROM space was feasible and the improved routines became standard.