I have closed my current account/loan, how come there is any debt left? How did the debt amount get so high?

It is very possible that there may be some residual costs or expenses related to the resolution of your overdue claims that will continue to be due after the closure of your account or loan.  Such costs and expenses will be in-line with whatever original agreement was made with your original lending institution.  Furthermore, any legal collection costs or fees incurred in the process of resolving your claim will be added to your account.