For the cache bytes value, Reporter looks at every single logs line and scans for the TCP_HIT message, which is actually a message in the access logs that indicates that the traffic is served and cached by the proxy. When Reporter sees this, it takes the byte received value as the cached byte value.
Therefore, when Reporter does not see this TCP_HIT message in the access logs, it does not log in a value for cached bytes and displays it as 0.
Examining a raw log, below is the line that generates a cache bytes entry. (The TCP_HIT message is highlighted in red)
2020-02-31 02:13:00 255 10.10.10.10 - - - OBSERVED "Global_Auth_Exceptions;News/Media" http://news.google.com/nwshp?hl=en&tab=wn 200 TCP_HIT GET image/gif http www.gstatic.com 80 /news/img/bluelogo/en_nz/news.gif - gif "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 1.1.4322)" 22.214.171.124 4245 316 -