The documentation describes that Roll-up custom formulas apply "normalization".
The normalized value x for a field is defined as 100 * (x - min) / (max - min). where min and max are the min and max values respectively across all the ideas. Hence, the max value will be interpreted as 100 and the min value will be interpreted as 0.
I've checked roll-up with several different inputs, and never saw normalization applied. For example for a rating field:

Is this a bug in the formula implementation, or in the documentation?