Henry_Best:
I don't claim to understand your code, but this is how I see it.
You want a see-saw, but you can't have a see-saw that starts with both ends on the ground. There's no 'half way' (evenly balanced) with PWM, it's either on or off, up or down.
Code is based off
http://interface.khm.de/index.php/lab/experiments/arduino-dds-sinewave-generator/
I am doing SPWM to get a PWM with varying duty cycles. This in turn when put through a basic RC filter yeilds a nice sine wave.