In 1974 strata law in NSW was changed to redefine common property from being everything outside the mid-line of the external walls to everything outside the inner surface on the outer walls. Thus, windows, sliding balcony doors and entry doors went from being lot property to common property for buildings completed after that date.
Have a look at this post on the LookupStrata website which pretty much explains everything.
And, yes, establish the true age of your apartment block. And yes, ask the strata manager (politely) to explain their thinking.