It seems to me that the session data for one user (for example: applied filters, current page, last selected server etc) in the web monitoring is shared per user and not per browser session. So if you have two computers logged in with the same user (say, zabbix adminsitrator), they share these settings. So if one put a filter on items, the other will see the filter too as soon as he enters the same page.
More over settings are remembered even if browser sessions have ended (for example if you log out) and this is very annoying in my opinion