Apologies if this is a known issue, but I didn't find anything by searching. It appears that Piwik isn't handling users through proxies, or in particular, users that bounce around via multiple IP's. It appears that Piwik is able to differentiate between multiple users on a single IP, but a single user on multiple IP's seems to not work.
AOL is the main ISP that I know of that may send visitors to a site through proxy IP addresses that change. Right now, I have a visitor browsing via IP's such as the following (and numerous additional IP's) and Piwik sees the user as a different visit on each.
I'm a little surprised that AOL still does this as I would have thought they'd have moved away from this type of system. A number of years ago I had to implement a complicated patch on a very large website with numerous front end web servers for exactly this situation.
Well, it appears that Piwik isn't simply differentiating users by IP and there must be cookie or other data used as well (since it differentiates multiple users on a single IP address). Perhaps that logic could be modified to not break if a user is on multiple IP's?
You may be able to improve the detection by configuring proxy_client_headers and/or proxy_ips. (See config/global.ini.php.)
Otherwise, if a user is bouncing between IP addresses and deleting his/her first party cookies, then Piwik doesn't detect this as the same user. (To more reliably detect such a case would require more extensive browser fingerprinting as seen in the Panopticlick study.)