Each airfoil has inherent characteristics and therefore different pitching moments according to their center of pressure. A wing is not balanced at the magical figure of 25% because of the wing.....the reason why its because most turbulent flow airfoils have their maximum thickness around that point and generate there maximum amount of differential pressure at that point. The first airfoil shown has the center of pressure at 27% of the chord withing the MAC. Flying it at a CG of 35% makes it marginally stable but within the allowable for 3D flying. Now, on the second airfoil the center of pressure is a lot forward, 16%, if we moved the CG at the same 35% point we are going beyond the point of neutral stability into negative stability. Thats why the model behaved the way described by Kenny. Increasing the horizontal stab area just increases the effectiveness of the flying surface but the model will remain negatively stable.