I don't know what numbers are in any particular POH, but by definition Vx is the speed for the steepest climb angle, so unless Vy happens to be at the same speed, Vx will be steeper. The POH numbers may include fudge factors for expected pilot skill, averaging for varying conditions, etc. But I also see errors in your spreadsheet; for example, the formula in column J should be ATAN not TAN... not that it greatly affects the results at these small angles (as the tangent of a small angle is very close to the angle itself in radians).

I suspect that the POH numbers to clear the obstacle, from which you take the C-150 Vx climb rate (I didn't look further) include the time it takes to initiate the climb (accelerate the aircraft upwards) from zero ROC on the ground, i.e. the actual ROC at Vx is probably greater than you calculated from the published obstacle clearance distance. The faster the aircraft, the more ground you'll cover during that transition.