Okay so, you go to an enchanting table to disenchant items. Simple. You gain that enchant and can use it to enchant other items. Simple. You progress through the game and come across another item with that same enchant yet more powerful.

e.g. If you kill a target within 4 seconds you fill a soul gem.

You find a weapon that gives you 20 seconds yet you can't disenchant it to get that enchant when it's clearly more powerful. So I'm wondering if I'm missing something here or if it's set like that on purpose.

Your'e only destroying the object to get the idea of the enchantment. The actual strength of the enchantment depends on your Enchanting level.

I think you just get the effect. The magnitude of that effect is based on your level in enchanting / the perks you have put into it. Haven't messed around with it enough to be sure though :/

Ahh, that makes sense.