Probably because ATi might have just given XFX a worse batch of HD 5870s than HIS. I wouldn't really care though, because XFX has double lifetime warranty AFAIK. I'd get the HIS for that epic computer toolkit thing, that brush/light/pen IIRC.
Since one has far more negative reviews, there is probably some difference between the two. A lot of people were saying that the XFX causes their screens to go gray and that it fails or freezes on them. Don't see that on the HIS card. Also a big difference between numerous user reviews and a thorough review by a website that probably used the same card for the entire review.
The HIS card possibly be newer, the gray screen was mainly due to the older ATi Catalyst drivers, which were released during the launch of the HD 5800 series. If you really are concerned, then simply buy the HIS HD 5870. I'd buy the XFX for the warranty (double lifetime warranty) HIS for the bundle.
You must also take into account that XFX's warranty is modder friendly IIRC, which means that more will have tried to overclock on the XFX versions, possibly causing GPU stability issues or failure. The fact that the card is factory overclocked probably does not help, and also the XFX card has nearly double the reviews - which is probably a rough guess of double the sales, which means double the chance of getting a defective card. Besides, you have to account for the no. of BS newegg reviews, and reviews which do not relate to GSOD, and other trivial matters.
Seriously, I'm not surprised that the guy who recommended the GTS 250 over the HD 5870 suffered from such problems, possibly because he may have installed incorrect drivers etc. Also, the guy that said his card was not factory overclocked, it's possibly a mistake from XFX, or he was looking at the speeds during idle. Otherwise it wouldn't have taken much effort to overclock using ATi Overdrive which can o/c far past the XFX's XXX edition overclock. Some guy said he couldn't overclock past the stock speeds - not all GPUs overclock the same, and the airflow in his case could have been poor, causing stability issues with any overclock.
Just a few examples of newegg reviews. I personally wouldn't stop buying a card simply because of say 20% of reviews said it had GSODs which were resolved, and stability issues which underclocking can cure. Besides, did I mention that XFX has a double lifetime warranty?
I bought a Radeon HD 4650 XFX model w/ 1 gig ram and replaced a 5 year old 6600 GT w/ 256 ram and experienced a dramatic framerate loss. I went through the online support and while team was polite, I found them to be condescending, not respecting my experience, they suggested common knowledge and were downright flat on ideas to fix the problem. I would expect a team with as good a reputation as XFX to have original ideas and be willing to examine the situation from a unique point of view. I don`t believe there was a technical fault to the card mainly based on their confirmation that the 3dMark06 score being "more or less where it should be", I believe there was something wrong with the support, especially their unwillingness to examine any system info I provided. I tossed the card in the trash and re-installed the 6600 GT (and have been less frustrated) until I find a better card. Believe me, I will do more research this time. I would recommend to everyone to stay away from XFX.