Two domain browsers in 1 domain

In my domain there two NT servers.
The first server is running Windows NT 4 with SP6a and installed as PDC.
The second server is running Windows 2000 Advanced Server and installed as member erver.
When I used Browser Monitor to examine the statusof browsing system in my domain, I discovered that both servers are browsers.
This surprised me very much because as far as I know there must be only one muster browser in domain.
Any suggessions?