GeoNames Forum

Link:
http://ws.geonames.org/findNearbyPostalCodes?lat=43.695733&lng=-79.530336
gives Downtown Toronto (St. James Park) - M5C as the closest to the searched lat,long combination.

But doing a google search with same lat,long show that adress is
28-52 Braecrest Ave
Toronto, ON M9P 1Z6, Canada

Google reverse geocoding service gives 6 chracter postal code but geonames give only 3 character and that not too accurate.

Is there any specific reason for this?

For Canada the service returns the FSA, the Forward Sortation Area (the first three characters of the full postal codes). The main reason are license restrictions.
Another reason is that for an international service this corresponds to the granularity in other countries. Most countries have fewer postal codes by orders of magnitude (Canada has over 700000).

http://en.wikipedia.org/wiki/Postal_codes_in_Canada

Best Regards

Marc