Like apertures, common focal lengths tend to follow multiples of the square root of two.
So, 24 (close to 25), 35, 50, 70 (OK, that's not a good example as 85 is used instead), 100, 135 (close to 141), 200, 300 (close to 283), 400 etc.
TC's follow this pattern too.
I guess it all centres around the 'normal' 50mm lens, even though 43mm is a better fit for a 35mm frame.
As for the actual focal lengths and apertures of lenses, remember the specs quoted are usually a near fit for the actual real life specs of the lens (as is often revealed in the patents), so they tend to get rounded to the nearest fit.