I'd guess a combination of all the things mentioned above. That's a pretty steep prop for 21", btw.. I'd expect a larger diameter and less pitch. But since it is "spec" for the boat, subtract a bit for a dirty bottom, maybe a dirty prop, maybe a bit of RPM missing, maybe a bit off on your GPS, tach, add 2000 lbs,,.... 3 knots (if indeed it is that) may not be out of reach for the perfect boat.
With a new bottom and near zero in my water and fuel tanks, I can reach 9.4 knots in my fat Manatee at about 3650 RPM with a similar 140 HP Yanmar. Six weeks ago I left Stuart for Indiantown on the Okeechobee Waterway. It would still turn 3600, but with a year-old filthy dirty bottom and running gear, maybe 50 gallons of water and 200 of fuel, the best I could do was 7.8 WOT.
The IP PY Cruiser is still a heck of a boat.
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Larry
"When life gets hard, eat marshmallows”.
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