Headline swings like that usually come from a small sample rather than a real value decline. A neighbourhood with only a handful of sales in a given month can show a large percentage move simply because one lower-priced or lower-condition home sold, not because typical homes there are worth less.
Mix shift causes the same effect. If the homes that happened to sell recently were smaller, older or in rougher condition than the ones that sold the year before, the average or median moves down even though comparable properties are worth the same or more.
The only way to know what your specific Mayfair home is actually worth is a valuation built from real comparables, not a headline number. Get your hand-reviewed valuation from Joel Dyck.