Problems with polnish postal codes

Yahoo placefinder has selectively problems with polnish postal codes:

Certain valid postal codes cannot be found (eg for postal-code 03-290):

http://where.yahooapis.com/geocode?postal=03-290&country=poland&locale=pl_PL

returns:

{"ResultSet":{"version":"1.0","Error":0,"ErrorMessage":"No error","Locale":"pl_PL","Quality":87,"Found":0}}

whereas neighboring postal codes are found (eg for postal-code 03-291)

returns:

http://where.yahooapis.com/geocode?postal=03-291&country=poland&locale=pl_PL


{"ResultSet":{"version":"1.0","Error":0,"ErrorMessage":"No error","Locale":"pl_PL","Quality":60,"Found":1,"Results":
[{"quality":60,"latitude":"52.302645","longitude":"21.039760","offsetlat":"52.307308","offsetlon":"21.039280","radius":3000,"name":"","line1":"","line2":"03-291
Warszawa","line3":"","line4":"Polska","house":"","street":"","xstreet":"","unittype":"","unit":"","postal":"03-291","level4":"","level3":"Warszawa",
"level2":"Warszawa","level1":"Mazowieckie","level0":"Polska","level0code":"PL","level1code":"MZ","level2code":"","uzip":"03-291","hash":"","woeid":12858771,"woetype":11}]}}


What is the reason for this problem?