...and a frequency of 975.9 Hz.
You forgot the current limiting resistor, which may have damaged all of the I/O pins that you attempted to connect a LED directly.
Have you set the pins to be an output before using them?It sounds like you have not if you are only getting a 2.7V peak output. On the other hand have you measured the +5V with your scope to see if it is calibrated.The other thing is that you could have too much of a load, like an LED without a current limiting resistor.
How are you measuring/calculating that? Since it's about twice (half the period), it looks like the right timing for a single-pulse or a half-cycle. i.e. If your 'scope has a built-in frequency counter, it may not be triggering properly.
Remember th3 default pwm state is not perfect wave to say, it counts up until the numb3r and flip flops, another state which is slower but more accurate is where it counts up and then back downSearch arduino secrets of pwm for a better explanation