Trouble with the Cooling Power of Fans
The power rating of the fan only tells you just that: the power rating of the fan. There is more to heat transport than how much power your fan uses (wastes). This includes:


How much energy is wasted pushing the air through the fins
How large the thermal resistance between the peltier and your cooler is
How large the thermal resistance between the peltier mounting point and the heatsink fins is, including the non-linear behaviour of the heatpipe for the temperature at which you are working


These define how well you can get rid of the excess energy from your peltier element and what temperature it will see on its hot side.

A few notes on things you didn't ask:


Using the cold side temperature to gauge how well the setup works is a bad idea. It depends on too much you haven't under control to be actually useful. A better way is to keep the cold side at a specific temperature (close to what you intend it to be used) and measure how much heat is transported at what power consumption of the peltier.
The peltier's surface should be covered by the cooler. Leaving spots open will cause the peltier to form hotspots at these uncovered parts and slowly degrade its performance. If the mounting area is too small, consider adding a 3-5mm copper plate (or 10mm aluminium) of the size of the peltier to ensure that no hotspots will form.
Please use proper SI units when discussing engineering topics. We do not use our hands and feets for measuring anymore.