To be honest in my opinion last 2 years matters the most because the main core subjects of the department are mainly in last 2 years.In 1st year all the subjects are same for every engineering student i.e. Engineering graphics,Maths 1,Maths 2 etc. They either calculate your grades of last 20 courses taken or last 2 years.
Also which subject you chose for doing master's also consider what subject grades they may count first.Suppose if you are going for M.Eng. in Mehcanical engg. with specialization in Thermal Engineering.They will also give special attention to the grades of the subject related to Thermal engg. i.e. Engineering Thermodynamics,Heat and Mass transfer etc etc Maybe if a person with 66% have good grades in such subjects related to the specialization area so they have been selected.
This is what my research says.I am no Expert or something.But I do have researched a lot for mater's in canada by meeting people,agents and from the internet as well...