CI doesn't really work how you might logically expect. It's a direct link between how much you spend and what the CI will rise to, simple as that. You can run the best, most punctual airline in the world with a totally modern fleet, good fares, frequency etc. but your CI will peak at some fixed level depending how much you spend on marketing. If you stop the spending it'll begin to drop - right back down to about 20% I think.
It's not linear either. To get from, say 70% to 90% will require a certain weekly budget but to get from 90% to 100% will take several times that again. It depends on the number of destinations you serve, I believe, but for example, I'm currently spending just over $5M per week and my CI won't go above 90%. I haven't tried, but I suspect I'd have to double that spending to get to 100%.
It'd be nice, and much more realistic if just being "good" would raise your CI but it just doesn't work like that.