Question about CustomErrors

Dear all I'm facing a strange behavior with customerror configuration 
In my website I've configured in web.config the following parameters:
<customErrors mode="On" defaultRedirect="/htm/errore_o.htm"> <error statusCode="404" redirect="/errore404.htm" /> <error statusCode="500" redirect="/errore404.htm"/> </customErrors>
<httpErrors> <remove statusCode="500" subStatusCode="-1" /> <remove statusCode="404" subStatusCode="-1" /> <error statusCode="404" prefixLanguageFilePath="" path="/errore404.htm" responseMode="ExecuteURL" /> <error statusCode="500" prefixLanguageFilePath="" path="/m/GesErr/error.asp" responseMode="ExecuteURL" /> </httpErrors>

When I call ...