I have seen what you describe when the domains are "parked" together, or if two sites are using a shared hosting web server.
Even if you don't see this problem, Alexa is not a good tool to determine what the traffic will be for any domain, especially one that has expired. Alexa only shows traffic from Alexa users and if someone with the Alexa toolbar loaded goes to the site over and over again it can raise the traffic score after a while. On the other hand Alexa generally shows you much less than what the traffic really is.
And any domain that has expired is going to start losing links and traffic, so when you get the domain you may not get much benefit from anything the domain used to have.
I would start with
https://siteexplorer.search.yahoo.com/ and see if the domain has any links to it. If it does, then you can assume that the more links it has, the more traffic it will be getting (for now). In the case of zillernet.com there don't seem to be any links to it.
