ive interviewed at uwo twice in the past two years and heres how they work:
they look at your two best years, provided you have taken 5 credits (100% load) in each year. you need a minimum of 4 credits for a year to be considered for calculation, but you will be penalized for that 4 credit year.
additionally, western will wait for your final year marks to come out, and if your final year is one of your best years, they will consider it among your best two.
also, if you apply during your 5th year (the so called 'victory lap' year), western will average your scores as follows, i believe:
1st year: remains as it is
2nd and 3rd year: averaged together so as to have ONE cumulative percentage
4th year: stays as it is, but since 2nd and 3rd year are avged together, 4th is now your 3rd year mark
5th year: stays as it is and becomes your 4th and final year mark.
Uoft will drop your worst year, provided you apply AFTER you have graduated, since uoft needs at least 3 years worth of marks and they release all their decisions before your final year marks come out.
hope all of this helps. people aiming for western should make it a point to have their honours degree completed by 4th year. i was told by the adcom that i had been waitlisted both times because i did not have a degree (i decided to go for a victory lap in 5th year). honours degree holders get a 5% bonus added to their final average.