It is sad that most companies claim 98%, because there is really no truth behind those fact. If you ask them for the actual numbers most will not release them. We decided to climb with E-Trip Africa as they do put up statistics that seem a little more realistic. They claim 88-89% overall, and when I asked for actual numbers of people it calculated correctly.
When I asked them about that, they gave a real nice explanation that helped me understand how things work. The companies that are targeting budget minded travelers usually use the Marangu Route (Coca-Cola) route. This route has very bad success rates (40%-50%). Some of the reason is because you progressively gain altitude and don't "climb high and sleep low". The other theory is that people going on a budget trip might not be as serious and don't prepare enough physically, mentally, and equipment wise.
Some companies really push clients away from Marangu. We were convinced that if we were already spending a few thousand for the flight and trek, it was better to add a $300 extra to actually have a realistic chance of reaching the summit. All 5 of us made it.
Take a look at the stats on E-trip Africa website. You will see a link to them on the right hand column of the pages about Kilimanjaro.