I've checked this perk "effect" which could be found with the Extra Effect perk.

It seems to work only with Set Value (as in Extra Effect), because in any other case you cannot select more than one enchantment.

My best guess is that this is another example (alongside with magic effect archetype) of a sloppy hard-coded element:
- You cannot find anywhere how this [censored] thing works.
- It is hard-coded, so you cannot see how it works.
- Bethesda didn't care explaining how this worked.
- It works in a way which is against any logic (if you can set a value, you should be able to multiply it as well!)

However...does anyone has any idea of why this happens?
Jashkar