Forgive me Velo, but you really have too much time on your hands if you're noticing that.
As a pro writer/journalist, it's a technique I would use to vary my language and avoid making the text monotonous - Clancy is a master at making the text light and easy-to-read, I really try to learn a lot of that whenever I re-read one of his books.
I've been through all of them - there is no other author that comes even close to his style.