It seems that the formula used to calculate NP doesn’t take into account that the stab operates in disturbed air, so the NP will be further forward than the calculation suggests.

For example, if the tail has an area 20% of the wing, and the tail arm is 2.5 times wing chord, the calculation would put the NP at 67% MAC giving a static margin of 41% if you choose the CG location at 25% of MAC.

I prefer the empirical calculation NP = 25 + 40*Vbar (as a % of MAC) where Vbar is the tail Volume ratio.