No one is guaranteed a particular length of authorized stay. It is likely that a B1 holder with a need to be in the US a short time would be issued a more limited authorized stay than one with a legitimate need to stay longer. B1 is more typically 3 months.
B2 visa holders who have a pattern of frequent, long visits and EOS, are often issued short stays as well.
The length of authorized stay is determined by the IO at the border based on his evaluation of the circumstances of the visitor.