This isn't any kind of weird mystery . . . it's simply a matter of Staff and Spear being related via Form Mastery, not Weapon Adaptation. Form Mastery reflects real-world styles that use distinct grips that
GURPS would call "staff" and "spear" grips. Weapon Adaptation, on the other hand, represents real-world styles that use unorthodox weapons for a particular
GURPS skill and never teach what
GURPS considers the orthodox skill/weapon matchup. It's a judgment call, and the perks are similar in some ways, but there are both real-world and game-mechanical differences.
Anyway, just about all Spear/Staff styles – and many Polearm/Staff styles, and a few Polearm/Spear styles (the three skills inter-default) – include Form Mastery. It isn't automatic or universal because (1) not everybody who learns to use a weapon one way learns to use it both ways, and (2) even those who learn styles that teach fluid shifts can have preferred modes and/or not be very good at those shifts. Still, the perk is present in most such styles.
MCMAP is an exception because it's an
unarmed style, and Spear and Staff are
optional skills picked up via training that commonly occurs alongside MCMAP but that isn't part of MCMAP proper. Those skills aren't in fact taught for use with the same weapon. Spear is there for rifles with bayonets. Staff is there for
pugil sticks, which "simulate" rifles with bayonets, I guess, but not especially well. There are no fluid shifts.