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You've got something installed that's splitting taxes into multiple subtotal lines ... somewhat akin to the recently-posted SplitTaxLines contribution ... but which predates that contribution.

I...

You have pieces of functionality on your site (including tax-related things) that are not part of the default Zen Cart distribution code. I was looking for a list of things you've changed and...

What addons/contributions do you have installed?
How long has this mis-calculation been happening?

What's "in" your GST zone, if you click on the "details" button?

Suggestions:
- check your zones to ensure that you've actually added BC and AB to your BC and AB zones.
- try some test transactions yourself, using BC and AB addresses ... do taxes calculate...