Previous foreign income changes the amount of tax refunded

[Sb113]

I've moved to Canada back in September 2019 and started working in BC from October 2019 onward. Whilst filling out a Tax return i got a warning about foreign income sources prior to moving and when i input my income before,my tax refund got reduced significantly. I paid taxes back in the country i previously resided in and want to understand why i would be taxed more based because of it?  

Many thanks

[TurboTaxSusan]

You are not being taxed on the income you earned before coming back to Canada. Instead, what is likely happening is that you are no longer eligible for all (or part) of credits that use your income as part of their calculation/eligibility.