Why am I being charged interest on Apple Credit Card for iPhone purchase?

Why am I being charged interest on my new Apple Credit Card if i purchased an iPhone from Apple.com? It is advertised as 0% interest for 24 months if I use the Apple Card. I read some feedback on this forum as it's clear I'm not the only one experiencing this. Some feedback says that we are only charged interest if we are late with our payment. I paid my bill on time, so that can't be true. Another bit of feedback says that we only get charged interest on the remaining ballance. Well, that's not how this was advertised. The remaining balance is only for the new iPhone. I have made no other purchases, so the only balance is related to the new iPhone purchase, which should be interest free for 24 months. What am I missing? Please provide a detailed explanation! Thank you.