Three ideas come to mind as possible:
1) the longer life statement was in error
2) the battery is defective, but only in life expectancy
3) the battery is defective or the is some unplanned drain on the battery
If the battery problem is not detected, I think the battery life should be checked often, at least every six months.
All that said, I have no idea how they determine the battery life left; One has to know the manufactured capacity and the drain current and how long the drain has existed to computer the life left.
I have a pacer and the way they tell how much battery life is when they readily your pacer. I've never heard of a battery going that fast..... I've had a pacer clinic tell me my battery was going to go in 2 years when it still had 6 years. That might change when she gets it checked next time, either that, or she has a bad battery.
Ages ago, St Jude has a software problem with detecting the battery life. I think they asked the physician download the new software already.
Mine battery was flat and my EP don't know. Was discovered by another hospital when I under a pre-check operation.
Your mom's pacemaker was most likely programmed incorrectly which may have prematurely drained the battery or if its a manufacturer problem, pacemaker has 3-4 year warranty. You should find a good EP doctor. They are hard to find in the Philippines. My dad was lucky. He has a good doctor that did his ablation.