Bank reconciliation is comparing the Bank's records of what happened in reality with Odoo's record of what happened in reality.

Both have: dates, amounts, and a journal with an account.

An unexplained difference occurs when unconsidered differences exist.

Traditionally, I have discovered examples such as:

• reconciling a payment in Odoo with a bank payment occurring BEFORE the date it occurred in Odoo (bad recording date in Odoo - the bank can record on the same day or on a subsequent day, but never BEFORE the payment date in Odoo);

• reconciling a payment recorded in one bank journal with a bank statement for a different bank journal;

• bank statement lines reconciled with Odoo payments that are then set to draft;

• bank payments reconciled with an invoice or bill instead of the existing payment;

• missing bank statements;

• missing bank statement lines;

• gaps between the ending balance of one statement and the starting balance of the next statement;

• manually created journal entries directly journaling to the bank account instead of using AP or AR with a corresponding payment;

• incorrectly recorded opening entry for the bank account.
  
  

Use date filters and the "bank statement line reconciled with this entry" field to find payments (journal items) in the bank journal that have not been matched with a statement line, check the journal of the bank statement line matches the journal of the entry, etc.

Note: not all of these will result in a balance in the unexplained difference section (depending on the version of Odoo you are using) but all are causes for problems caused my not correctly matching the two sources.  In version 14, this has changed with the use of the outstanding accounts and suspense accounts. See https://www.odoo.com/forum/help-1/v14-change-in-payment-behavior-how-do-the-suspense-and-outstanding-payment-accounts-change-the-journal-entries-posted-177592 for details.