Cherubs?

In Cliffs Of Dover the altitude is sometimes given as “Angels” (thousands of feet) and sometimes as Cherubs. Whats this? Is the first time I hear it. Hundreds of feet??

Found it here:

http://sierra-kilo.com/Blog%20-%20Angels%20and%20Cherubs.htm

“Angel” describes altitude increments of 1,000 feet and “Cherub” describes altitude increments of 500 feet