GW8.0.2 client/server

1 - from GW8.0.2 client, I logs into my Caching Mailbox, and receives all emails
2 - then logs into Online Mailbox, and select emails and 'Archive'... done, i.e emails moved from my Online Mailbox to Archive.(I verified this by connecting to Archive)
3 - All those emails I selected for Archive in Online Mailbox are also not available in Caching Mailbox.

why ? when I only Archived the emails on Online Mailbox, why those emails are also moved from Caching Mailbox ?

Please help