TCP applications like PING prefer to resolve names by DNS. MS networking commands prefer to get names by WINS/broacast and other NETBIOS functionality but each method will eventually use the others resolution if it's preferred mode fails, it'll just take a while to get there.

There are (from memory) about 8 different ways to resolve host names, the only difference between NBTSTAT and PING is the order that these methods are used.

Fo instance I've just pinged my NETBIOS name "RICHARD HOWARTH". Now, I know it is not a valid DNS name, not least because my IP address is DHCP assigned and there is a space in it, but after a short delay the PING command happily resolved the name and pinged away.

Your warning should therefore be something like:
PING -A returns the DNS name by preference. If there is no DNS name PING may return the NETBIOS name.
NBTSTAT -A returns the NETBIOS name by preference.

I'm unsure if it is possible for NBTSTAT to have an unresolved IP address in it's tables.