PATH is an environment variable, and therefor it 'defaults' or 'resets' when you change environment. See
man sudoers for an explanation:
env_reset If set, sudo will reset the environment to only contain
the LOGNAME, SHELL, USER, USERNAME and the SUDO_* vari-
ables. Any variables in the caller's environment that
match the env_keep and env_check lists are then added.
The default contents of the env_keep and env_check
lists are displayed when sudo is run by root with the
-V option. If sudo was compiled with the SECURE_PATH
option, its value will be used for the PATH environment
variable. This flag is on by default.
is the basic path without modifications. Different users will have different directories added to them for several reasons.
And the reasoning should be: root should never ever have more directories in its PATH than needed. Or the other way around: if root needs a file it should be in
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin. You do not play games as root. You do not use a desktop manager as root. A root is for admin tasks.
The wiki has some more information (amongst others which files are used to add to PATH): https://help.ubuntu.com/community/EnvironmentVariables