Why do Planning Poker cards deviate slightly from the Fibonacci sequence?

We’re anal software people, too. We know that the Fibonacci sequence should go from 8 to 13 to 21. We use 20, though, instead of 21. Of course we used 21 in the early days, but dropped it after meeting with a product owner who looked at the 21 and said, “Oh, a 21. You must be very confident to give such a precise estimate. Most people would have called it 20 or 25, but you called it 21.” We replied that we used the Fibonacci sequence. The product owner said, “The Fibo-what?” After which, we decided that deviating from the Fibonacci sequence would be better because the numbers started to imply a very high level of precision that just wasn’t there.

The 1/2 is included because after awhile some teams decide they want some room in between “free” (a zero) and one. Using 1/2 isn’t something a team should do a lot, but it’s a nice option to have in some cases.