inconsistent upstream_addr log

My log format looks like this

log_format vcombined '$host $remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for" '
'$ssl_cipher $request_time $gzip_ratio '
'$upstream_addr $upstream_response_time $geoip_country_code';


I get these logs

www.example.de 192.168.1.1 - - [04/Apr/2013:13:41:58 +0200] "GET /de/test/ HTTP/1.1" 504 383 "-" "Mozilla/5.0 (Windows NT 6.1; rv:20.0) Gecko/20100101 Firefox/20.0" "-" RC4-SHA 120.045 - 10.6.10.10:81, 10.6.10.11:81, 10.6.10.13:81, 10.6.10.12:81 30.003, 30.000, 30.002, 30.001 DE
www.example.de 192.168.1.1 - - [04/Apr/2013:13:41:58 +0200] "GET /de/test/ HTTP/1.1" 504 383 "-" "Mozilla/5.0 (Windows NT 6.1; rv:20.0) Gecko/20100101 Firefox/20.0" "-" RC4-SHA 120.045 - 10.6.10.10:81, 10.6.10.11:81, 10.6.10.13:81, upstreamgroupname, 10.6.10.12:81 30.003, 30.000, 30.002, -, 30.001 DE
www.example.de 192.168.1.1 - - [04/Apr/2013:13:41:58 +0200] "GET /de/test/ HTTP/1.1" 504 383 "-" "Mozilla/5.0 (Windows NT 6.1; rv:20.0) Gecko/20100101 Firefox/20.0" "-" RC4-SHA 120.045 - 10.6.10.10:81, 10.6.10.11:81, 10.6.10.13:81, 10.6.10.12:81, upstreamgroupname 30.003, 30.000, 30.002, 30.001, - DE
www.example.de 192.168.1.1 - - [04/Apr/2013:13:41:58 +0200] "GET /de/test/ HTTP/1.1" 504 383 "-" "Mozilla/5.0 (Windows NT 6.1; rv:20.0) Gecko/20100101 Firefox/20.0" "-" RC4-SHA 120.045 - upstreamgroupname - DE

I can understand the last and first log line. But what is happening in between? Why is the upstreamgroupname and the end or in the middle?