I think that's not irrational.
Important to understand that this is not a developer proposal, it's the council setting out broadly what it would allow, should a developer seek to develop the site.
If they say 9 storeys max, then certainly any developer will try and push for a bit more. There might be others reading this who will know more than I about how these "Site Allocation Development Plans" tend to pan out in other places where they have been adopted.
Also, I don't know whether the SADP has any specific time period on it. For example, might it get reviewed in 5 or 10 years, and the decision made that it would be ok to edge it up from 9 to 11 because it's not that much extra?
This is one of the reasons I'd prefer that these decisions about height be more related to a set of general principles, with some reasoning behind them. Because then it's easier to prevent that incremental increase over time - get one thing accepted then apply for something just ever so slightly bigger, once everyone is resigned to the first version. One thing is daylight which is why I brought it up before. If we agree that there is a minimum level of daylight that anyone should expect to remain available to them when they choose to live in a certain place, then that is measurable, and a proposal either does or doesn't cross the threshold. Instead we seem to have a minimum that it turns out can be chiselled away using dubious use of recommendations made for a different purpose. In which case, what's the point of having the minimum standard in the first place. Another measurable thing is relating building height to street width. That is a logical approach with lots of precedence.