As they and most municipalities have done for many years the interim tax bill is 50% of the total tax assessed in the previous year. "As property tax is a large obligation relative to other (non-financing) home ownership costs, cities typically want to ensure residents have enough time. However, budgets for the year (and subsequently, tax rates) are not typically financed until the end of the first quarter of the year. Because of this, the city usually issues two bills: an interim bill and a final bill.
The Interim Tax Bill estimates the taxes for the year and issues a bill for
half the amount. Later in the Year (usually by May), the municipality sends the final tax bill, approximately equal to the interim bill, plus or minus the adjustment difference between the estimate and the actual tax rates for the year." See"
https://catalystre.ca/toronto-property-tax-info/
The interim bill really has no relationship with the final bill we will all get in May as it does not take into acount any changes in the MPAC valuation or the tax rate for the current year.