I noticed something really weird about insmod..

Code:
$su
Password:
VCC_SRC_HOME = /root/VCC2
[root@sarayu tipc_demo-1.15]# which insmod
/usr/bin/which: no insmod in (/root/VCC2-Deps/java/jdk1.6.0_01/bin:/root/VCC2-Deps/ant/apache-ant-1.6.2/bin:/root/VCC2-Deps/java/jwsdp-1.5.0/jaxb/bin:/root/VCC2-Deps/JacORB/JacORB_1_4_1/bin:/root/VCC2-Deps/aspectj/aspectj1.5/bin:/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/bin:/usr/bin:/bin:/usr/X11R6/bin:/home/chaitanya/bin)
[root@sarayu tipc_demo-1.15]# exit
exit
and
Code:
 $su -
Password:
VCC_SRC_HOME = /root/VCC2
[root@sarayu ~]# which insmod
/sbin/insmod

In the first case , i logged in as su without the -l option and insmod failed to work. In the second case, with the -l option, it worked. What is going on here?