People really count miles they planned to run but didn't?? That's asinine.
I would agree with what the other poster said that people tend to state the highest week they ever did, or maybe the highest they hit on a semi-regular basis and state that as the average.
I think that this is part ego, part wanting to show off, and part not wanting to let the competition know what you are doing. Runners/coaches tend to be like chef's, or fishermen, they never want to give away exactly how they did something, just vague innuendos intended to hide the truth, like the way they do it is some super secret formula.
Also, stories tend to get exaggerated over time, it's like the telephone game. I remember one year in high school as one of the better runners in my conference talking to my coach at a meet about a particular week during the summer where I had gone a little overboard. I was finishing up one run and my legs started to give out underneath me, all wobbley and weak, so I started to walk, and it wasn't any better, so I just sat down for a little while, and then walked home. This was a one day occurrence and I promptly backed off and recovered. Anyway, apparently another runner from a rival school overheard this, and pretty soon this was what I had been doing all summer. I was later approached with this and had no idea what the other person was talking about, until I remembered the conversation with my coach and connected the dots.
On the other side lots of runners understate their mileage. That was the way my college program was. We wrote mileage in a similar fashion to the Badger mile philosophy. A lot of times during the summer when I'm going out and running 5:30 to 6:00 pace every day with no workouts I would write down 75-85 and it was in reality much closer to the 85-95 range. The highest I've ever logged is 105, and I've only written down 100+ twice, but that 105 week was probably 115, and I've written down lots of 95s that were really 105. Why? I have no idea.
Training post-collegiately I knew lots of people who hit 120 miles or so on occasion, but maybe one or two that broke 100 week in and week out.
I guess the bottom line is that to take anything people say about their training with a grain of salt, unless you know them really well and actually are familiar with what they do.