A number of teams have created additional numbers to be used in estimates because they do not think that the set that are provided are enough. Remember that what we are doing is relative size estimates and so the numbers from 1 to 100 represent 2 orders of magnitude in size and so should be enough to determine how much will fit into a sprint.
The need to put additional numbers in seems to happen at both ends of the scale:
The research indicates that if you limit your discussion to a sequence of numbers, you don't spend a lot of time arguing over the exactness of the estimate. Further by having the numbers spaced out, you can get to “its a 5 or 8” pretty quickly, and then you can decide one way or the other, again reducing the amount of time wasted in doing estimates.