It's all some sort of approximation. You certainly noticed that not all months are just 30 days long, nor are 52 weeks a full year (~1.25 days missing, that's already on average worth 180 blocks). Your 144*30=4320 is also different from my estimate of around 4368 blocks per month. Doesn't matter, we're roughly in the same ballpark...

The crucial point is: if you're interested in mining, DO the math!