After wasting too much of my time learning about MOSFET device physics, I think that I've figured out why your setup was working. When the MOSFET is off, it drops the voltage in the circuit "before" the gate -- this means that applying 5v from your uC acts as 5v between the gate and the source, turning the MOSFET on. Once on, the voltage dropped in the MOSFET disappears, so your 5v would be below the required drive current, right? Actually, the Gate-source capacitance keeps the charge there, and the uC won't allow the charge back into its pin, so when the MOSFET "floats" up to 6v, because of the charge in its capacitance, the Gate floats up to 11v! Then, when you set the pin on your uC low, the MOSFET sees that as -6v Gate-Source, turning it off almost ideally.
You've inadvertently created a psuedo-bipolar power supply for your MOSFET, Admin!
P.S. Let me know if you find this description reasonable -- feedback would be appreciated.