Your grades will be verified by the time registration happens, or possibly even if/when you firm the university. Otherwise, they aren't that bothered about processing your grades if you're not taking the offer.
Interviews are good, so yes, as above, in an ideal world it would be nice. It isn't practical however.
The exams/assessment process of pre-university qualifications probably should be tighter, to get over the "they're truly intelligent and not just very good at remembering and regurgitating stuff in exams." part. I don't think it needs "passion" to be successful, technically. An interest in doing well/some interest in the subject is about all that's necessary. I like my subject, but I wouldn't ever have considered myself passionate (and right now i'm rather looking forward to finally finishing my degree and getting away from most of it).
If you've got grades in hand already it's not really surprising you get quick offers through - your grades meet or exceed the requirements, your PS isn't complete rubbish, and you've shown some vague possible interest in going there by applying. They're not necessarily expecting you to take the offer(s) [especially when your grades are already much higher than the requirements] so they're not that bothered about chucking them out. Now they effectively can take as many as they want over certain grades it's not really that much to them.