Hi,
that is done for all modifications. The modification is always 1/1000 mm "bigger" than you have set it.
This is build in and cannot be changed. The reason is that if thios mnwas not used and you would have object touiching each other you for sure would get collisions (very small ones) because of calculation inaccuracies.
HTH,
Christian Blei
CBT Christian Blei Tools
christianblei.de
youtube.com/channel/UCxjA_NbeScQy9C0Z1xjwXpw