I believe you prepared a joint return? Between the two times you saw different numbers, did you add or change anything else in the return? even something small like a $5 donation, a medical receipt, or a transit claim?"
Even tiny changes in total deductions can cause the "Optimizer" to recalculate the ideal split from scratch, which often shifts the refund/owing balance.
On the Pension Splitting page, is the 'Optimizer' currently set to 'Maximize Refund,' or have you manually selected a specific amount to split?"
If it's set to "Maximize Refund," the software will constantly search for the best penny-perfect split. Only setting it to "Manual" or "I want to choose my split" will lock the numbers.
When this happened last year, did the CRA say the amounts on your return didn't match the amounts on your spouse's return (Form T1032)?
