Yeah, usually its the short trips + using the audio that does it. Here during winter, I will only get like 1 crank from the battery on super cold days (-20F). So, thats why I switched to the 51R.
Also, I would have somewhere else check your battery as well. Sounds unlikely that its 'perfectly fine'. Autozone does it for free. Might just be good to get a second opinion. Check the terminals as well. Make sure they are clean, tight, etc.