The thing is even until a building is topped out heights can change. I've been trying to get the height for the Grand Hyatt down in San Antonio, and even though the building has already started construction and is actually halfway up, there's a chance the penthouse may be shorter if it's deterimined they don't need a parapet wall for the mechanical stuff.
I also usually wait until a building has begun construction to even ask for the heights since plans are usually a little bit more finalized by then. Even so, I'll check back with them later on to see if they're the same. Floors can be added, or removed, and the little things like an extra couple of feet from a barrier wall, parapet, can add or take away height.
As for the heights being so close to each other, yet the buildings have different floor numbers, it all depends on what the use is. Again, this is why I always wait until they're started and then check back again later. It takes 2 years to finish a building. A lot can happen in 2 years as far as the economy goes and even just normal tweaking of the design. Skyscraper developers are creatures of survival.

If one thing isn't working for them with one set of plans, then they switch to make it work. Just look at Tom Stacy's planned tower at 5th & Congress. Other examples would be the Four Seasons Residences and Altavida, which have existed in one form or another for nearly a decade or more.