There are subtle effects that can result in a teetering rotor from producing exactly 90 deg phase lag. For example, any of the following will result in a deviation from an exact 90-deg phase lag:
- pitch-flap coupling as a result of a delta3 angle
- friction/damping in teetering hinge
- aerodynamic effects like modifications to blade lift as a result of dynamic operating conditions (varying angle of attack, etc.)
- the rotor hub being underslung
- etc.
Of course, any phase lag should be considered in relation to the blade pitch being injected by the swashplate. There are all sorts of geometric/mechanical scenarios that can give the pilot the impression of a phase lag (i.e., flight control mixing, bellcrank geometries, etc.), when in reality the phase lag might be very small.