My iPhone 5S (iOS 7.1.2) battery went from being 19% to 30% just as I plugged it in?

My phone battery was at 19% so I plugged it in to my cable (mophie 3 metre/9.9ft lightning cable) and it immediately jolted to 30% in about a second.Is my cable made from unicorn horn or it just a bug?

Battery is failing, software problem…