As Bigfred and others so eloquently pointed out there are many variables, including temperature, wind, weight loss, and how you've been eating/hydrating that can change the averages of each individual ride. I know I've averaged as low as 10.6 on the Garrett County Masochistic Metric (63 miles with 8327' of gain) to a high of 21.2 on a short 11 mile flat-ish (52'/mile) loop I do here.
I only compare me with me.
Keep track of what you do and try and improve on it.