Bharat Sanchar Nigam [BSNL] — incorrect landline deposit refund

I had disconnected my landline in sep 2019 and broadband in 2014.in september 2020 i received a partial refund. I do not know why the deduction was done inspite of my having paid all the bills on time till date and not a single late payment during the entire period. I met the jao / clerk and he actually didnt know the reason. I would like to know the reason for not paying back the full deposit refund.