Real estate agents (in Alberta at least) have to pull the condominium plan registered from land titles and use the listed offical square footage - not "estimate" on their own. Note that even the "official" square footage is subject to interpretation, as it can include other space registered on title, such as the parking space, storage locker, and even balcony space.
I'd love to see Developers adhere to these rules too.... I can personally attest to having bought my first condo (in my naive pre-Stratasense daze..) on the basis of the square footage presented in the marketing materials, and I'm pretty sure it didn't live up to it's 850 sq ft billing....
Still not sure about the square footage listed on title? If you are in the pre-purchase phase, consider hiring a professional measurer and having them do their thing as part of the "property inspection". If you're able to catch the seller and their agent juicing the numbers, it could be a pretty powerful negotiating chip to save you some dollars.