In the same area you'll also find Paradox Essences, which will either increase or decrease this number depending on which one you use.
Most likely this will range from -5 to +5. So if you want to find an Enigma armor (which will be a unique) and it's on #445, but you found a #430 relic, then you can use Essences you find in the area to decrease it until you get the exact number you want. Note however that you can only do this a certain amount of times, so one relic can only yield a specific group of items as a result!
The items will spawn on high requirement weapons & armors, exclusive to Relics, you need to cycle through and find a good unique/set/runeword (that are now technically uniques) to makeup for the high requirements. Affixes such as crushing blow, slow and absorb will be replaced with something else.
In addition to this, I may place older versions of items I make into the relic as well. Say for example I nerf or buff an item to give it less HP, the item will retroactively be unuseable and require you to place it in the cube to provide the newest version. The previous version will then be added to the relic list.