Why is the language encoding in X different to what it is within the TTY console?

For example, if the system language encoding is set to zh_CN.GB18030 by entering the following into /etc/sysconfig/i18n:

LANG="zh_CN.GB18030"

After logging into X, the language may be set to zh_CN.UTF-8 as shown by the following:

$ export|grep LANG
declare -x GDM_LANG="zh_CN.UTF-8"
declare -x LANG="zh_CN.UTF-8...