Why are capacitor voltage ratings such strange values?
It's mainly all about preferred numbers: -



See this wiki page and, see also this stack exchange answer about fuses values.

It's all about splitting the difference between 1 and 10. For instance \$\sqrt{10}\$ is ratiometrically half way between 1 and 10 and the cube root splits into the numbers 2.154 and 4.642. Multiply by 2.154 again and you get 10.

So, if you split the range 1 to 10 into 5 chunks you get 1, 1.5849, 2.5119, 3.9811, 6.3096 and finally 10.

These values approximate to 1, 1.6, 2.5, 4.0, 6.3 and 10.

Like I said earlier, this is mainly the reason but I suspect the prevelence of 24 V systems may have caused the 35V capacitor to come about and, of course the 40 V capacitor is not that uncommon: -