Why did my result show "more blades on turbine = more RPM"?

In the following, I will assume that the only thing that changed, was the number of blades of your test turbines. Thus, all other parameter of your turbines such as swap area, blade geometry, height, generator, etc. will stay constant.

Blades of wind turbines use the same aerodynamic principle as airplane wings do: By generating a pressure difference on both side, lift is caused. Since the blades are forced to move on a circular path, lift is transformed into torque. Hence, incrementing the number of blades, also increments the torque $M$ and the power $P$ since: $$ P = 2 pi n M quad,$$ where $n$ denotes the number of revolutions. As stated in the comments above, the more blade a wind turbine has, the less is its efficiency, but this may be neglected for small numbers of blades.

The resistance of the model turbines (As the only difference is the number of blades) will only increase with an increase in revolutions until equilibrium is reached, explaining the increase in revolutions as it was observed.

The same applies for real wind turbines. However, for wind turbines it may be favorable to keep its number of revolutions constant or even reduce them as excitation frequencies are couple to them by $$ f_{1P} = n qquad text{and} qquad f_{3P} = 3n $$ due to imbalances in the rotor ($f_{1P}$) and tower shading ($f_{3P}$). Eigenfrequencies of wind turbines are typically at 0.2 Hz to 0.3 Hz, what exactly is between those two frequencies for three-bladed wind turbines with a rated rotational speed of 10 rpm. In order to avoid resonance, the number of revolutions should stay roughly the same or even decrease. Said that, on increasing the number of blades you may also increase the generators torque in order to keep the number of revolutions constant.

An other possibility could be the fact, that by keeping the power constant (for various reasons) you can reduce the number of revolutions by the same amount you increase the torque. Despite the effect of reducing the excitation frequency with the number of revolutions, noise pollution (blade tip speed) and load on mechanical parts such as bearings and gears is reduced as well.