For many countries lat/lng are determined with an algorithm that searches the place names in the main geonames database using administrative divisions and numerical vicinity of the postal codes as factors in the disambiguation of place names. For postal codes and place name for which no corresponding toponym in the main geonames database could be found an average lat/lng of 'neighbouring' postal codes is calculated.

For Canada we have only the first letters of the full postal codes (for copyright reasons)

For Ireland we have only the first letters of the full postal codes (for copyright reasons)

For Malta we have only the first letters of the full postal codes (for copyright reasons)

The Argentina data file contains 4-digit postal codes which were replaced with a new system in 1999.

For Brazil only major postal codes are available (only the codes ending with -000 and the major code per municipality).

For India the lat/lng accuracy is not yet comparable to other countries.

Reference http://download.geonames.org/export/zip/