[akeefer]

That's a great, succinct way to put it. It pretty much exactly matches my experience, too: out of every 10 things I estimate, a couple go a little faster, most of them are within 10-20% of the estimate, and one or two of them blow out to 2-3x the estimate. (And of course, if I knew which ones would blow up ahead of time, I'd revise the estimates upwards initially so that didn't happen . . .). If you have 6 projects to do that you estimate at one month each, you need to give yourself closer to 9 months to get it done if you want to have any chance of finishing on time, because it's a near certainty that at least one of those projects is going to blow up on you.