iFixit iPhone 4S Replacement Battery Shutdown issues

After using my iPhone 4S for 3 years, its original battery was terrible. It was draining faster than any electronics devices battery. I replaced it with iFixit's battery. After some initial success in battery life, iFixit's battery started behaving the same, with some slight(!) issue. When iFixit's battery in the phone, the phone shuts down sometimes at 25% charge, sometimes at 30%. I can't even make an estimation on when it'll shut down. I don't see any solution to this, so I will switch back to my old battery, but does anybody know why this early shutdown thing happens? Am I the only one who's having this problem?