QBO Payroll Canada asks for employee totals as of June 30 and current YTD. Why is the June 30 split required? What is it used for?

Why does QBO Payroll require employee payroll history to be split at June 30? (Canada) During the payroll history setup, QBO asks me to enter two amounts for each payroll item: -Total as of June 30 -Year-to-date total For employees who were paid after June 30, this requires entering cumulative totals as of June 30 and then entering updated year-to-date totals for the same payroll items. I'm trying to understand why this split is required, not simply how to complete the screen. From what I can tell: -Payroll tax calculations (CPP, CPP2, EI and income tax) are based on year-to-date totals. -T4 reporting is based on annual totals. -Ontario has no payroll rule changes effective July 1. -Journal entries and financial reporting are unaffected by the historical payroll data. So what purpose does the June 30 snapshot serve? Is this: -required for some internal QBO payroll calculation, -used for migration validation, -or simply a design choice in the import process? Has anyone entered the same YTD totals in both columns and experienced any issues with payroll calculations or year-end T4s? I'm looking for a technical explanation rather than "that's just how the wizard works."