Is there a reason or code behind the Cessna model numbers?
There is no theme, but like some cars or mobile phones, there's a logic to it. A higher series (200 vs. 100) is bigger, a higher sub-series (180 vs. 170) is more powerful, the P and T are pressurized and turbo, and so on.

The no theme is supported by the Apr 1981 issue of Flying Magazine, where it is confirmed by the Marketing SVP of Cessna (p.58).



Re: what about the last digit, e.g., 180/182/185?

"Engineering numbers" mean internal numbers with no marketable meaning, just like the first two digits. My guess is that there were 181, 183, and 184 designs on paper at one time.