Why when my phone's battery low it drains very quickly?

[The Strange Tester]

Hello everyone. Yesterday I forgot to charge my phone, it had left about 15% left. When I was out, I checked the time and put it away, after some short time, I checked it again and it had only 1%, but it lasted long, I even turned down brightness. So the point is here: Why does my phone battery drain very quickly when it's low?
My phone is Samsung Galaxy Trend 2