A couple of points,
If she was recorded as being 70 at her death, she presumably looked 70, or not much older, I had a great aunt who died in her 90’s who looked not much older than her 70’s.
As others have said, Dad’s Mom can be confusing, was the writer married with children? The writing could be from the children’s perspective, not the writer.
Does the other photograph look at all like this? People can change in looks very fast, there could just be a year between the photographs?