To answer your question directly, Sir Stanley, yes, that is how is is designed. Presumably less chance of Mr Up-cock if the 'default' setting is the one you expect to be using (e 1013.2/29.92) when you are actually flying with that set. If a 'random' QNH setting (eg the last one used) was allowed, there is always the possibility it might not be noticed, whereas 1013 does tend to be 'noticed'. Additionally if an inadvertent selection of QNH is made above TL there will be no 'false' input to the autoflight system.