It might be noted that the increase in magnification that occurs here can be significant.
On 1000mm focal length, a
5mm = 200x
4mm = 250x
3mm = 333x
The increase from a 5mm to a 4mm is 25%, and the increase from a 4mm to a 3mm is 33.3%
Given that the eyepieces will likely yield high powers in the scopes they are used in, the last jump in particular might jump right past the seeing ceiling of the night.
A 3.2mm would have yielded the same % increase from 5 to 4 to 3.2mm, and a a 3.4mm would have yielded the same magnification increase from 5mm to 4mm to 3.4mm.
Now I can see why both a 3.2mm and 3.4mm eyepiece have existed in the last 10 years and where those odd focal lengths came from.