Let's say your figures are correct and using a slip of about 13-18%, which I reckon is likely, then an explanation is that your prop is not at the pitch you think it is. Although, it may be correctly marked.
I have no experience of what I'm about to suggest but I can't see why it's not a possibility.
Turbulence can form a barrier which can behave almost as a solid. Codprawn frequently recommends Volvo trim tabs (although, apparently, having not yet fitted his) which are plates that project down into the water parallel to the transom. They work by causing an area of turbulence at the rear of the hull which reprofiles the hull into a downward hook. The faster the boat travels, the firmer the hook.
I see no reason why a prop blade with a pronounced cup or blade curvature could not form turbulence within the hollow of the curve which would then behave as a solid and increase the effective pitch of the blade as its speed increased. It would be a neat trick if it worked because it would behave as a variable pitch prop.
Your prop would need to increase from 25" to about 28-30" to make your figures work and that doesn't seem beyond the bounds of possibility and an unreasonable increase to me.
But, of course, I could well be talking bollox as usual.