Some gages have internal damping - a small restriction that smooths out the needle movement. With fewer cylinders a six can have more manifold pressure fluctuation than a V8.

 

There could be a bit of variation in sensitivity among gauges, for instance some gauges have a very tiny supply hole which is designed to dampen the signal a bit. Also the size of the vacuum hose and its softness can also have different dampening effects, and affect how much the needle will twitch.

Edit: I didn't see the Flyer's post before writing this one.