Why don't our numbers match between systems?
Because the systems count different things with the same words: timing differences, duplicate or wrongly merged records, refunds handled one way here and another there. It is always findable: trace, explain, repair, reconcile, then automate the join so it stays fixed.
The longer answer
In practice.
The usual culprits
Date boundaries (order date here, payment date there), records merged on the
wrong key, deletions in one system invisible to the other, GST in one total and
not its sibling, and the manual export somebody edits before importing. Every
mismatch has a mechanism; "the systems just differ" is a surrender, not an
answer.
Where the discipline comes from
Eleven years of records and systems work in places where numbers being wrong
has consequences: mining, policing, federal government. The habit transfers
whole: trace the mechanism, prove the repair, reconcile against a source you
trust. Yours is almost certainly smaller than it feels.
What done means
A written explanation of the mechanism, repaired records, totals that agree
with a source you trust, and the automated join that prevents the rot returning.
Method at data & reporting.
Same rabbit hole