Grouping - strange behaviour

[markus.weisser]

Hi, i've created a formula in order to show always the previous record number in the current recordset row,
when the record number changes.

It seems to work, when i place both fields - the  current number and previous number in the detail section

Example:

Current  - previous
40 - 40
40 - 40
41 - 40
41 - 41
41 - 41
41 - 41
42 - 41
42 -42
.....

The strange thing now is, when i place a grouping on the previous formula field to the report,
the engine creates two identical groups for instance for the number 40.
In my example the first two records with identical current number are showing up in grouping with 40...
The change in line 3 creates a new group showing 40 as well... !?!?

Wy does the engine create a new identical group?