It differs because everything except saline and albumin is filtered out. The point is to show that some markers of aging are harmful byproducts in your blood, and filtering them out can have positive effects.
Actually, albumin seems to be filtered out as well and replaced. If I understand correctly, albumin is a transport protein, so it carries around a lot of stuff. When it gets replaced, it is replaced with a "blank" version.
I would say that it's unsurprising that our organs are less than perfect at doing their jobs, given that they operate under various constraints (e.g. running under an ATP power budget, one tuned for an ancestral environment where dying of hunger was a real concern; being only able to eliminate metabolic waste at the rate blood flow can take it away; having to avoid strategies that lead to DNA damage, like running anaerobically for long periods; etc.)
Therefore, it's also unsurprising that external devices can do some of an organ's jobs "better" than the organ itself can. They devices don't have the same constraints our organs do.
This effect is already very clear for pacemakers: hearts are very bad at keeping themselves going in oxygen-poor conditions (as they effectively have a circular dependency—a heart can't beat without flowing oxygenated blood); but electronic pacemakers don't need oxygen, and so keep the heart going under such conditions, until oxygen-rich blood can return.
Once we perfect stem-cell organ synthesis, it'd be intriguing to know whether "untethered" human longevity could be increased just by sticking "moar kidney" and "moar liver" into the body, than the human body-plan calls for by default.
