Sundry can mean various, miscellaneous, or diverse. Sundry debtors might refer to a company's customers who rarely make purchases on credit and the amounts they purchase are not significant.
I suspect that the term sundry was more common when bookkeeping was a manual task. In other words, prior to the low cost of computers and accounting software, a bookkeeper had to add a page to the company's ledger book for every new customer. If a new page was added for every occasional customer, the ledger book would become unwieldly. It was more practical to have one page entitled sundry on which those occasional customers' small transactions were entered.
With the efficiency and low cost of today's accounting systems, I believe the need for classifying customers and accounts as sundry has been greatly reduced.