Difference between Value Line Arithmetic Index and Value Line Geometric Index?

[pirx]

Hello!

Can please some explain what is the difference between these two indexes? Here´s a chart:

http://finance.yahoo.com/q/bc?t=1y&s=^VAY&l=on&z=m&q=l&c=^VLIC

As far as I know, both indexes move by same percentage points, but this arithmetic and geometric thing confuses me

 

[Trdr]

Pirx, happened to be on the 'Gummy' site yesterday so here's a link:
http://www.gummy-stuff.org/zweig.htm