Rasbpian Wheezy has been end-of-life for quite a while so new installs should use Stretch. Stretch is supported with updates and security fixes from Raspbian for quite a while. As to the DNS IP, yes, some routers will round robin addresses and some clients will query all available DNS servers and return the first response. There is a configuration that will force dnsmasq to use strict ordering, but the clients do not always follow that.
Take a look at Why should Pi-hole be my only DNS server? to see some more information on that.