I assume it's somehow related to a builtin function running to figure out how to evaluate the "end"?
Yes. The expression o.test{1}(1:end) is equivalent to
temp=o.test{1};
n=numel(temp);
o.test{1}(1:n);
So, subsref is called twice. If you had done o.test{end}(1:end), then it would have been called three times.