They're both areas covering Aberdeen City, but there might be what could be classed as historic towns now included in them - Peterculter, Cults, Woodside, Bucksburn, Newhills, etc which were all outside the city before various boundary changes in 1891, 1930s, 1960s, and then the big one under regionalisation in 1974.
What time period are you looking at and what towns were you expecting to see? The best way to familiarise yourself with the area is to follow the useful links provided by Paul S. and play around on the National Library of Scotland maps site.