WHY HIGH STAYS CONSTANT and low doesn't?

I read this article so far, and it stated that the High will always stay constant, while Low will decrease in bandwidth. i don't know if this is something only for the RC, or is it some general laws.

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Read the article: http://arduinobasics.blogspot.com/2014/06/433-mhz-rf-module-with-arduino-tutorial.html

Its utterly arbitary, ask the author of the article if you really want to know. The point is to show signals can be sent....