Don't they see the income each year to see if you qualify or not ?
Say this year you might have say $ 28,000 or less, but next 4 years you might have considerably more due to say a PT job, rental income, Investment etc . So someone who is approved for 5 years can cheat the system and get away with it, if they don't have to qualify each year.
or maybe the electricity guys automatically call CRA and check your income each yr with your SIN etc.