A quick look through Google found me a small article on EK's brilliant website which I've shamelessly copied and pasted. After all, don't fix what ain't broke.
What’s the difference between 520, 525 and 530 chains?
The numbers indicate a chain’s dimensions, tolerances, minimum tensile strength and other specifications, using a system instituted by the Japanese Industrial Standards Committee. The JIS standard is very similar to chain standards developed by ISO, ANSI, DIN and others.
The first digit (4, 5 or 6) denotes a chain’s pitch – the center-to-center distance between chain pins. Originally these numbers specified pitch in eighths of an inch (i.e. a 400-series chain had a pitch of 4/8, or 1/2 inch), but now metric dimensions are used. A 400-series chain has a pitch of 12.7mm. A 500-series chain has a pitch of 15.875mm. A 600-series chain has a pitch of 19.05mm.
The second and third digits indicate the chain’s width, measured between the inboard surfaces of the inner sideplates. An EK chain ending in ‘20’ measures 6.35mm between the plates. If the number ends in ‘25’ or ‘28’, the dimension is 7.94mm. If the number ends in ‘30’ or ‘32’, the width is 9.53mm.
Because wider chains are typically stronger as well, the second and third digits also indicate a chain’s strength, relative to other chains with the same pitch. Higher numbers correspond to greater strength, i.e. a 525 chain is stronger than a 520. This additional strength is achieved through increased roller diameter, pin diameter, pin length and plate thickness.