Well, Argus believed that the talented users of Argus cameras should take advantage of the high precision of the C3. Therefore, they incorporated shutter speeds of less than a "stop" of exposure apart. This allowed the highest degree of exposure precision to be harnessed with the perfectly timed shutter. The constant alteration of the shutter speeds showed Argus' continued dedication to quality improvement over a long range of time (remember, the C3 was in production for well over 20 years).
Okay, now in all seriousness, there are probably several reasons for the "tinkering" of the shutter speeds. The very first Argus C3 cameras came out with more shutter speeds than later models. The really early C-series cameras (like the Argus C, C2, and early C3's) even had a 1/5 sec. shutter speed. My guess is that the elimination of a few shutter speeds was to decrease redundancy (eliminating such closely spaced speeds that you mentioned), allow a more regular progression of shutter speeds, and too possibly reduce production costs. So, this resulted in 10 shutter speeds at first, then 7, and ending with 5.
Also, have you ever seen the mechanics which operate the shutter? The design lends itself to a large array of possible shutter speeds, since it consists of a simple cam on the back of the shutter speed dial and two connecting rods coupled by a small lever. These are attached to the shutter escapement. Since the speeds are controlled solely by the cam, any possible number of shutter speeds can be configured by the shape of the cam, so changing the number of shutter speeds and their values would be an easy affair for Argus.
In finality, remember that other camera manufacturers (such as Leica) were just as guilty of tinkering with shutter speeds.
