What are some reasons for big difference in time usage and small difference in mAh?

What are some reasons for big difference in time usage and small
difference in mAh?

I have found an android phone with really good specs like 1.5GHz Quad
Core, 1GB RAM, 5", HD screen. My question though is on the battery
Battery Size: 2000mAh
Usage Time: 6 Hours
Battery Standby: 72 Hours
In comparison with let's say Samsung S4, it comes with a battery of 2600 mah,
Stand-by (2G) / Up to 370 h (3G)
Talk time (2G) / Up to 17 h (3G)
Why is that big difference in the time of usage with just 600mah? I got
the numbers for the S4 from Gsm Arena and the other one from the
manufacturer.
Thank you for your time