The LDS church counts every baptized member. If someone has fallen away but never officially resigned, they continue to be counted until the age of 110.
They assume by the time someone reaches their 110th birhday they most likely have since died…
It was another one of those odd things that keeps their numbers inflated.
Im not sure what the world wide membership is at now so I will guess 13million.
They have a poor retention rate for adult baptisms. Vast majority go inactive by the time a year has past.