Cherokee 180 prop RPM

I have a '71 PA28-180, with a fixed 60" pitch Sensenich prop. This is
the standard cruise prop for a PA28-180.

The POH indicates that 2700 RPM should only be able to be achieved
through 7000ft DA.

I am able to hit redline (2700 RPM) in level flight up to 11500 feet
density altitude. POH indicates that it should achieve about 2550.

It has operated this way as long as I can remember (several years) so
it is not a new phenomena.

So what could cause this? Prop pitch, poorly calibrated tach? Any
others? My plane has all of the K2U speed mods, so the reduced drag
probably plays a role.

I am able to achieve close to 150mph true airspeed at max cruise,
while the POH indicates 140mph for book. This makes me think the prop
RPM indications might be accurate.

Appreciate your thoughts.

Thanks,
Nathan