Path>Difference : Resulting zones do not match

Hello,

I am working on a logo (containing, among others, a Turtle and a Moon) and I would like to cut off the silhouette of the Turtle into the Moon so that both object will be on the same level (as pieces in a jigsaw puzzle, rather than one on top of the other). I eventually want to create a SVG document that would be printed in SPOT.

When I do Ctrl+D followed by Path>Difference, the Moon does get punched out by the silhouette of the Turtle BUT the edge of the Moon object does not exactly match the edge of the Turtle object : at places, both objects are set apart by a few pixels, letting the background show ; at other places, both objects are a bit superimposed (cf. following image showing a detail with Turtle in transparent red to show superimposition : http://1.bp.blogspot.com/-W2C5JrZY2Ig/UQAKg_4Y4mI/AAAAAAAAGTw/qY62vLsh7a0/s1600/path4113-8.png )

When I saw this problem for the first time, I thought it was due to me slightly moving one of the object while selecting them. However, while investigating more closely, it appears that the edges of both objects do not match, even by slightly moving one object to the left, right, higher, lower...

The screen-print above was not in full zoom in order to show both problems at once and even if at normal size (x100) the issue is almost invisible, one can still feel that both objects do not match : the touching edge is not as sharp as if the objects were on top of each other. See images :
- Path>Difference : http://3.bp.blogspot.com/-5eCUt5A0JLE/UQCoPmbeBJI/AAAAAAAAGU8/xpugOUKSUCA/s1600/path2.png
- Objects on top of each other : http://3.bp.blogspot.com/-cZyDkZ7jmI0/UQCoPsHAkSI/AAAAAAAAGU4/w1IaM0TmwGU/s1600/path1.png

I believe that it is not a big issue, but I am surprised that the Difference tool does not not cut out exactly along the edge of the top object. What would be a way to solve this issue ?

Thanks,
Yvan