Why is __FILE__ uppercase and __dir__ lowercase?

Why is __FILE__ uppercase and __dir__ lowercase?

References

Reference NO 1

http://stackoverflow.com/questions/15190700/why-is-file-uppercase-and-dir-lowercase

Reference NO 2

https://www.ruby-forum.com/topic/193674

Reference NO 3

http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Variables_and_Constants

Reference NO 4

http://zenspider.com/Languages/Ruby/QuickRef.html

Reference NO 5

http://dougireton.com/blog/2013/03/17/a-knife-dot-rb-for-our-time/

Reference NO 6

http://phrogz.net/programmingruby/language.html

Reference NO 7

http://www.perlmonks.org/?node_id=995692

Reference NO 8

http://neurogami.com/content/10_minutes_to_your_first_Ruby_app/