You've told MATLAB that you want fill element 10 of the array a with the output returned by a call to the constructor with no inputs. MATLAB also needs to know what you want elements 1 through 9 to be and it determines the latter by making a call to the constructor with no inputs, as stated on this documentation page. It uses that "default" element of the class (from the automatic 0-input constructor call) to fill in all the elements up to element 10, then uses the one you explicitly created as that tenth element.
Actually, checking back I see your object is a handle object. In that case, see this other documentation page which goes into a bit more detail about how that is handled. Same general answer, but with an extra detail or two specific to handle objects.
