Is there some kind of variability between what a recipe says an item will be and what the final product is? For example I created a sword, the recipe said it would be +7 bleeding and the final was only +6 bleeding.

Intended or bug?

Probably a bug. A few people have noticed this, myself too. I think one person reported that it's a vendor display error. I have not really tested it in-depth, myself.

I just know my robust yellow was 1 point off on both minimum and maximum from one time I looked at the recipe.

I kind of assumed they are bugs, though it's also possible something can affect your chances when creating an item to add some variation to created items?

I've had several weapons that ended up with different damage ranges than the diagram stated, for instance. :/ Usually fairly minor, like 1-3 higher points in damage so instead of a listed damage range of 21-25, it'd become 22-26 for instance.

These aren't bugs. Some items simply have partially-random stats - for example, a sword might have Bleeding (6% through 10%).

The only 'bug' part is really that the crafting recipe doesn't show that it's a random amount and instead just shows a specific amount.

My Items and recipes change stats a lot. Same for vendors. So I belive it's a bug.

Bugs are simply undocumented features, don't ya know. ;)