Hi Duncan, thanks for your reply.
yes, they are being cut "for real": the polygons' areas are not equal (nor close to equal). See the screenshot of the attribute table: polygon2, which is the blue one at the bottom left corner, as an area half the size of the others in the same layer.[ATTACH=CONFIG]26704[/ATTACH]
The extent varies with the datasets (blue and pink buffers are cut at different extents), so it seems right that the extent has been set on the tool environment settings.
Is there any way to change that ?