The square of a planet's orbital period is proportional to the cube of the length of the semi-major axis of its orbit.
If it were precisely true, the entries in the last column here would be IDENTICAL. They clearly are NOT.
In the text that follows, "E" means power of ten.
Planet Orbital Orbital Orbital distance squared
Name Radius Period days divided by period cubed
Mercury 5.79E+07 88 3.990E-20
Venus 1.08E+08 224.7 3.986E-20
Earth 1.50E+08 365.2 3.984E-20
Mars 2.28E+08 687 3.982E-20
Jupiter 7.79E+08 4,331 3.976E-20
Saturn 1.43E+09 10,747 3.933E-20
Uranus 2.87E+09 30,589 3.971E-20
Neptune 4.52E+09 59,800 3.885E-20
^^^^^^^^^ these are NOT identical.
You are a good guy, and I like you. But you MUST understand that intuition is a lousy guide to Physics and Astronomy. The curvature of space is as certain as the curvature of the surface of the Earth.