Completely normal. When you close a program, the OS keeps the data for that program in RAM. (This is categorized as "inactive" RAM.) If you open the program again, a lot of the data it needs is already in RAM, so it opens quickly.
If you run out of free RAM, the OS will overwrite that inactive RAM with new data because currently running programs obviously take precedence over previously running programs. At that point, the apps will take longer to open.
OS X does a very good job of memory management on its own.