Fuhgedaboudit

I have a question to ask...today at the leake I noticed 1 of my 3 mercury 200 hps has water pressure slightly higher than the other 2 at 5500 rpm. the 3 of them used to have 15 psi exactly all the same...today my middle engine has 18-19 psi at 5500 and the other 2 still have 15. the temps are exactly the same, nothing has changed except the pressure. Last wednesday we ran this boat all day long and the water pressure were all even. Can anyone tell me if this sounds like a problem and /or why the changed today? at idle they all have even 2/3 psi and at 2000 rpm they all have 10 psi. Its not until you run at 5000/5500 rpm that the pressure is different. any input would be appreciated.