The best tutorial on this topic, and a must read for anyone who wants to understand this, is http://www.cambridge...era-flash-2.htm
If you just want to know the answer to the original question:
In Aperture priority mode with the flash forced to be on, cameras aim to achieve something close to balanced (1:1) exposure where half the light is from the flash and the other half is ambient. To do this the camera first measures the ambient light and calculates settings that give a 1 stop underexposed image. It then fires a very short pre-flash to determine the correct flash pulse duration needed to bring the total image up to the proper exposure level. [Note: In program mode a similar thing happens but most cameras treat it as a fill flash and produce a much weaker flash contribution than 1:1.]
To get a more dominant contribution of the flash to the overall exposure you can either go to manual mode, set ISO/aperture/shutter to values that give a more-than-one stop underexposure based on ambient light and then let the flash add whatever is needed to bring it up to the correct exposure level.
It is possible to define a different target exposure ratio than the default balanced exposure but you better read the link above for the details. Basically, to get a larger flash contribution you set the Flash Exposure Compensation (FEC) to a positive number. But at the same time you need to reduce the normal Exposure Compensation (EC) to prevent overexposure. I have never played with this but if the correct combination of FEC/EC to get 2:1, 4:1 etc ratios doesn't depend much on the ambient conditions then it could we worth investigating. If I get to it, I will report back.