you’ve estimated with Planning Poker, you may very well have used cards with either the Fibonacci sequence, or a modified Fibonacci sequence.

The traditional Fibonacci sequence is 1, 2, 3, 5, 8, 13, 21 and so on, with each number the sum of the preceding numbers.

Years ago I began having teams estimate with a modified Fibonacci sequence of 1, 2, 3, 5, 8, 13, 20, 40 and 100.

Why?

It’s because numbers that are too close to one another are impossible to distinguish as estimates.

i think using T-shirt size or just a normal sequence 1,2,3 or any even/odd series should be giving the right estimation if team has good technical and functional skills...but don't understand why most of people says follow fibonacci...

'How useful do you think most people would find it, when estimating, to assert that something is (say) 39 points, or 40, or 41? Is such precision likely to be realistic, when compared to estimating small work items of 1, 2, or 3 points in size?'.

My interpretation of the Fibonacci sequence has always been that as the uncertainty and complexity of the task at hand increase, so does the figure resulting from the sequence. The points increase significantly relative to an increase in complexity and uncertainty.

This means that when we assign a low amount of points to a task, we are more certain of the context, difficulties and attributes of that task than when we assign a high number. In your example, assigning in the range of 40+ points means to me that the task at hand is considered to be so large as to making any accurate approximation of that task impossible. T-shirt sizes might be the better choice in this situation. Tasks in that range should be refined to more comprehensible sizes in my opinion.