Newbie question about pkg_add

[Canhua]

Hi, good day all. I am new to FreeBSD.
I tried to pkg_add -r a package (py-networkx), which tell me that:
Error: FTP Unable to get ftp://ftp.freebsd.org/pub/
FreeBSD/ports/i386/packages-7.0-release/Latest/py-networkx.tbz:
File unavailable (e.g., file not found, no access)

although I know that py-network does exist in /usr/ports.
Actually I could go to /usr/ports/math/py-networkx and make install
using ports means.

Then I could learn from this that there are softwares that could be
install from ports while not able to be added from package system?
Am I right?