A different tyre pressure problem

bnmuk

I recently fitted a Tyre pal system to my caravan, on the wheelarch it says 4-5 bar (65psi), i set the range from 60-73 psi, on a recent long motorway journey on a warm day the warning came on at 73psi, i thought 73 was a little high, i know the tyre pressure
rises when travelling but didnt think it rose by that much, is this normal or do i have a problem, the tyres are the original tyres, 5 years old,would love to know what others think

Alan