Well... That makes it harder to add a level shifter. 
It could be normal part tolerances since I think you're on the hairy edge.
If the 5V regulator is adjustable try bringing it down to around 4.5V, or at least make sure it's not over 5V. Powering the LED strip closer to the 3V data signal may help. If it's not adjustable try a diode in series to the LED strip to get a small voltage drop. (With 150 RGB LEDs it should be rated for 10A or more.)