I would recommend reviewing your Schedule CA (540), line 2. If you had interest income reported on your federal return, it would show up on this line under column A. However, if it was interest income that was not taxable within California, it should show up on Column B which would explain the reduction in income on your state return.

Typically, your withholdings will only change based upon the input in the program. You should review your source documents to the amounts reported as state withholding on your California Form 540, side 3, line 71, to make sure they match.
If you do notice a difference, you will need to amend your return to make any corrections.
If you do not see the income reduced on Schedule CA, then it is possible that the tax withheld amount changing for the same amount is a coincidence.
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