Its probably just the GDDR 5 causing the issue. Some makers quote the actual clock speed and some quote the effective clock speed. GDDR 5 is basically quad pumped, technically its not quite that simple but for the purpose of this its close enough so the effective clocks are 4x the actual clocks.
Yep that's it, don't know where you are looking but if you look online at New egg under 4890 1GB you will see some write it one way some do it the other.
The marketing guys think the bigger numbers will sway people
Sapphire cards are phenomenal, I've never had any issues with the ones I or my friends have used. From what I've read (granted it was back in the days when the 9800 Pro/XT were the top of the line, now they're just epic legends) Sapphire is one of the closest partners to ATI. Not sure how things are after the merger, but my 4870 X2 from them has never had any issues.