Fixed charges are a big part of that gap. A utility or property tax bill usually includes a base delivery or service charge that does not move with usage, so a percentage increase applied on top of that fixed amount lands harder in dollar terms than the percentage alone implies.
Two households can see the identical published percentage increase and feel it completely differently, because the one with a larger existing bill absorbs a larger dollar increase for the exact same percentage change.
For a homeowner budgeting whether to stay or sell, the dollar total matters more than the percentage on the notice, and CMHC's own guideline weighs total housing costs at roughly 39 percent of gross income for exactly that reason. Get your hand-reviewed valuation from Joel Dyck.