This is quite an odd case, but possibly worth to be documented:
The button "Create mean value sample" of a collection will open a new window with mean value curves created from your collection.
Then the normalized curve of that mean value sample is created as mean values of all the normalized values from all the members of your collection.
The green ring width mean value curve will be created as mean values of detrended (as selected for the collection) ring width data of the collection members.
If you then insert a zero ring (right-click on curve) that normalized curve above will be lost and a new curve is calculated from that green ring width mean value curve.
N.B. when you then remove that zero ring (shift right-click on curve) you have a slightly different normalized curve in front of you than immediately after you created your mean value sample.
If you run a correlation test the correlation coefficient may first be 0.51 but change to e.g. 0.49 after the zero ring manipulations.
I have never found this to be of any importance, though it may puzzle a user.