1) I could care less if somebody made a clerical error
2) I could care less if the average house price jumps 20% or drops 20%, on a YoY one-month picture, because that stat, one month, means nothing.
Perhaps large townhouse or condo projects closed and there were a lot of resales, driving the average price DOWN. Perhaps more large homes were sold that particular month for some reason, driving the average UP.
This average price stat has nothing to do with individual house values whatsoever. Nothing at all.
As for taking their reports seriously, I'm not sure anyone follows them as religiously as you.

Yes they are important, but specific market info about home values can come from realtors anytime.
I gave you two options for the anomaly:
1) MLS District Re-mapping
2) Clerical error, corrected... call and ask them
3) Different parameters used for the searches
That's all I can think of - I'm just an independent Realtor, member of OMDREB, TREB, and many other organizations I don't always agree with on everything. I appreciate you asking my opinion on this, really, that's all I can think of as I have nothing to do with their stat reporting team - let us know if you find out anything more!
(I have just emailed OMDREB to ask)