Query about connections to oracle by tnsnames

Question

I am newbie to oracle connections. I would really appreciate it if you could tell me why before I was able to connect to oracle by cmd using:

c:\chris> sqlplus
c:\chris> Enter user name : system
c:\chris> Enter password:

However now I can't connect using system, instead I must use system@orcl

c:\chris> sqlplus
c:\chris> Enter user name : system@ORCL
c:\chris> Enter password:

My tnsnames is:

TNSNAMES

PD: Thank you.