That's because we are a bit more honest. Most hosts put it knowing full well they cant do it. It only takes a single, poorly written script to take a server down. So when you have a shared hosting environment where you have a few hundred clients per machine....well, there's probably no way you can offer 99% and be telling the truth.
There are hosts out there that offer 100% uptime, and mean it. However, they are about $600 per month for 10 megs of space and a gig of bandwidth. Our network connectivity is 100%, but we would hope that most people are smart enough to know a host cannot offer 99.9% uptime when most clients are paying less than $20 per month.
98.9% is our guarantee, not our average. I wont lie to get business. Sorry. Many hosts will disagree with me, but 99.9% uptime guarantee is a lie, they cant meet it. You really think all the $1.99 hosts sitting at Rackshack can truly do that? Those hosts dont even know where their machines are, cant upgrade them, cant have them built to specs, have to rely on a third party to make their machines stay up....yet they can offer a 99.9% uptime :S
99.99% is like the "unlimited" myth. Anyone can slap that number on their front page, it's merely a sales tactic... In our case, it would probably now be safe to up that number, but prefer not too just yet. We have a new $15,000 external program in place that blocks such attacks as DDOS and others. It's been in use since about two weeks ago, and uptime has been almost 100%. One attack got thru a couple of days ago and one of our machines went down for about 5 minutes....but thats just one out of many attempts. The program learns, so the exact same attack cannot be successful again.
Anyway, in the case of downtime, most hosts seem to blame it on something not covered by their 99.999% guarantee anyway (like provider problems), so what good is it? We just spent $50,000 on new hardware, moved everything close to me for immediate on-site support, and have the new software in place....I can promise you that if we cant offer 99.999% uptime, neither can most other hosts in the same, lower, and even a little higher price range as ours...