A “Ritek G05 disc” just means any 8x RITEK G05 disc made by Ritek – either unbranded or produced for a reseller brand like Arita, Ridisc, Ridata, Piodata, Memorex, or whatever. Ritek may make a bad batch that doesn’t pass their A-Grade certification, but these could still come out of the factory as shiny silver unbranded discs, for example – just because it says RITEK G05 doesn’t mean it’s necessarily Grade A. Sure, lots of people have had great results with unbranded Ritek discs, or cheap Arita discs, or even the overprinted orange “Mountains and Streams” discs which are supposedly factory surplus – but it’s a crapshoot and you may get a bad batch that way.
“Ridata” is Ritek’s own premium brand, and these discs, while basically the same as any other Ritek G05 disc, have much better quality control, so you have a much lower chance of getting a dud. Plus, the manufacturer support for these is generally better – if you do get a bad disc or batch, e-mail them with the serial number(s) from the discs and they should help you out. Ritek is aiming to have a great reputation with their Ridata brand, so things like that should matter to them (especially if linked to a public thread on CDFreaks).
Ritek also makes some claims that Ridata discs are safe from counterfeiting issues here, but I don’t know if anyone’s actually seen any counterfeit Ritek discs on the market. (Most of the fakes I’ve heard about have had TY, MCC, and TDK media codes)