If you have an unexplained difference (missing statements) in a reconciliation report, one of the reason could be the date you use to look at your report.

For instance I have a difference of 300€.

The balance of the account 5120025 at the date of the 22/02/2019 is 4156.92. When I looked at the journal items on this account the total balance was 4456.92 because one entry was booked on the 01/03. When I changed the date to the 1st of March, I got the unexplained amount back to 0


Additionally, from the V12 code:

If the amount is non-zero, it could be due to

1.  some bank statements being not yet encoded into Odoo
2. payments double-encoded""")

#NOTE: anyone trying to explain the 'unexplained difference' should check
# * the list of 'validated payments not linked with a statement line': maybe an operation was recorded
# as a new payment when processing a statement, instead of choosing the blue line corresponding to
# an already existing payment
# * the starting and ending balance of the bank statements, to make sure there is no gap between them.
# * there's no 'draft' move linked with a bank statement