Why did the narrator call her grandmother avva in the chapter how I taught my grandmother to read by Sudha Murty?

very touching story about a 12 year old girl and her grandmother who is illiterate. sudha murthy's grand mom was illiterate. they (granny n sudha) preferred a weekly local magazine called karmaveera which came in kannada lang. and that magazine had a episode story named kashiyatre. granny was fond of it and his granddaughter would read out that story. one day, sudha had to attend a marriage party at her village. as usual karmaveera came but granny was not able to read a Word only she could watch the images. when sudha came home, she found granny tear-eyed. she asked her the reason behind it and granny told about what happened when she was in the village. granny asked her if she could make her able to read and write. at first sudha was surprised at this. then, she thought 4 one more time then, she agreed. sudha worked hard and so did her granny to secure their goal i.e. to make granny literate. then, goal secured at saraswati puja time. when the festival was going, granny came and touched sudha's feet and sudha was shocked at her behavior. granny exclaimed her that she was touching her feet as a student not as a grandmother. sudha too bowed her at her feet and touched it. granny gifted sudha a frock. sudha and granny were both happy at this occasion. sudha was the first tutor of her grand mom.