Sometimes it is because of an asymmetric feet or asymmetric posture or gait. But sometimes it is just because it starts in one leg and it will get to the other later.<br />
I have plantar fasciitis in both feet and I wish it was one foot only. You can find out more about plantar fasciitis treatment in: http://www.plantar-fasciitis-elrofeet.com/How_is_Plantar_Fasciitis_Treated.html
This is most often caused with someone who is athletic and uses one leg more than the other ie running always starting with the same leg or jumping while pushing off more on one side, or landing more on one side; someone stands a lot and tends to lean on one side more than the other, or also if your hips are out of alignment you may be turning one foot in or out more causing the tearing which is plantar fasciitis. Try stretching your gastrocs out (do the ball of your foot on the stair stretch, or the pushing the wall one leg behind you heel to ground stretch) one side is probably tighter than the other, also stretching the front of your foot and leg helps too, (you can do that by standing, curl one sides toes under and place your foot facing straight down with the knuckles on the ground, bend your knee on that same side so that you are placing a slight pressure down and forward making your feet look like a ballerina, if you have poor balance, hold onto a wall or chair so you don't fall and cause other injuries).<br />
Also! very important- put a tennis ball, golf ball, something hard you can roll your foot on by the side of your bed. BEFORE you stand or put weight on that foot you MUST roll your foot back and forth on that for a few minutes. This will help stretch the fascia out before you weight bear. If you do not do this you will continue to rip the fascia which will continue your inflammation cycle. Also freezing a water bottle to roll under your foot at the end of the day will help immensely. Stretch at least 3 times a day on your legs like i described before, rolling before weight bearing and icing. That should make a huge difference. :)
My feet are different sizes but both have plantar fascitis. So, I guess your situation is possible too. Get good shoes. I've had best luck with Birkenstock. You can get a foot bed to put into other shoes that is wonderful!