ulimit -a doesn't appear correct for nice value for a given user

Issue

In a RHEL6 server, we set nice value to "0" hard and soft in /etc/security/limits.conf. When I su - to the ID and type "ulimit -a", the nice value appears as 20. If I change it to 10 in the conf file, ulimit displays as 10. If I change it to -15 in the conf file, ulimit -a displays it as 35. Can you explain why this changes? It is happening on every RHEL6 server I tried.