PHP Curl sometimes gets a 404 error when loading images but it works with file_get_contents() why is that?

Sometimes I get a 404 error when trying to access a webpage via curl but if I use file_get_contents to access the exact same URL it works, any idea why it's like that?

Here is an example: My curl settings:

curl_setopt($this->ch, CURLOPT_USERAGENT, $this->UserAgent);
curl_setopt($this->ch, CURLOPT_AUTOREFERER, TRUE);
curl_setopt($this->ch, CURLOPT_FOLLOWLOCATION, TRUE);
curl_setopt($this->ch, CURLOPT_COOKIEFILE, $this->CookieFile);
curl_setopt($this->ch, CURLOPT_COOKIEJAR, $this->CookieFile);
curl_setopt($this->ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($this->ch,CURLOPT_VERBOSE,true);
curl_setopt($this->ch,CURLOPT_HEADER,true);
curl_setopt($this->ch,CURLOPT_ENCODING, "gzip");
curl_setopt($this->ch, CURLOPT_CONNECTTIMEOUT, 30);
curl_setopt($this->ch, CURLOPT_TIMEOUT, 30);

The request header with my firefox webbrowser:

GET /image.png HTTP/1.1
Host: ssl.website.net
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20100101 Firefox/13.0.1
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Connection: keep-alive
Cookie: some cookie info
Cache-Control: max-age=0

Response header:

HTTP/1.1 200 OK
Date: Wed, 20 Jun 2012 13:38:43 GMT
Content-Type: image/png
Transfer-Encoding: chunked
Connection: close
Pragma: no-cache
Cache-Control: no-cache
Server: '; DROP TABLE servertypes; --