I can understand it with Usenetserver, I believe they have a hard cap of 120 million headers per group which means, on a large group, not the full retention.
What tier of service do you have with Giganews? Their lesser service tiers are limited to 30 days.
What's your "Storage Age" set to in Newsbin? It has to be > 500 days to see more than 500 days of posts.
Something a clever person could do would be to sign up for the giganews free trial, download all headers from the groups so, you have 1100 days worth. Then use these headers on servers like Usenetserver that have 1000 days of files but, not the full retention for the headers.
In the options, network, click "Show Server commands" then right click this group and "Download Latest". In the logging tab, you'll see something like this.
[12:53:20] HIGH NNTPSocket - GROUP alt.binaries.tv
[12:53:20] HIGH NNTPSocket - 211 222914121 209186769 432100889 alt.binaries.tv
222,914,121 - Number of headers in group
209,186,769 - Start record range
432,100,889 - Stop record range.
What do you see?