Thank you for your reply guys.
The contractor who installed my security camera system first showed me the app on my smartphone. To ensure my iphone wirelessly connects to the live feed quickly, he had to set the live video quality to substream(D1), and the playback is just the normal 15fps quality, which gave me an impression that the playback quality is better than the live video quality.
It was not until hours later that I found out it is not the case on the DVR screen. Plus, the smartphone live video quality can be changed to mainstream, it will just take my iphone longer to connect. Anyway, main or sub-stream, 15fps or 30fps, the playback will never be as good as live. And the reason has already been explained in the previous reply.
So thank you, guys.