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.

Read the article: 433 MHz RF module with Arduino Tutorial 1

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