What's the strategy of DNS-SD services respond

Question

I am trying to gather some information of devices connected on the wifi with DNS-SD. When I use dns-sd -B _services._dns-sd._udp (OS X)/avahi-browse -t _services._dns-sd._udp(Linux) repeatedly to observe the published services, I get more or less output while devices(Mac and iPhone) are always connected. For example, this service(used by iTunes for Wi-Fi sync) shows occasionally while no one changes the sync setting of the iPhone.

Timestamp     A/R    Flags  if Domain               Service Type         Instance Name
12:07:40.042  Add        2   4 .                    _tcp.local.          _apple-mobdev2

So it's because the iPhone responses the mDNS query at intervals or something else? What's the strategy of DNS-SD services responding to mDNS query? How can I detect these services more accurately?