I suspect the entire income tax/NI system is designed to be as baffling as possible, in the hope that people can't work out just how expensive it is. The fact that income tax figures are stated annually or monthly, while NI is stated as weekly amounts makes absolutely no sense otherwise.
I can't be bothered to check the figures, but I think the point at which NI drops is equal to or around the point at which income tax jumps up. So people paying the lower NI are paying the much higher income tax, so are paying a higher overall percentage than basic rate tax/NI payers.
It's all smoke and mirrors, basically they want to take as much as possible without you being able to work out just how much it is. I've occasionally heard discussions over the years about simplifying it, but it never happens, presumably as they know there will be uproar if people actually see the entire lot as a single massive percentage.
There was also going to be another new tax, possibly named new NI or something, supposedly for social care, but basically just income tax part 4. Unlike normal NI, this would be paid by pensioners. I don't know whether this got scrapped, unscrapped or what during all the summer/autumn nonsense.