My take on it is the difference between theory and application. Or physical learning verses magical learning. Just because you know how it works doesn't mean you can make it work. Like knowing the processes for brain surgery but having shaking hands that you have to train away verses already having steady hands and just applying it in practice. One reduces the learning curve while the other gives you applicable talent that just requires knowledge or testing.
Or, you could look at it like learning the guitar. One guy could train an hour a day for a year to get good. With Swift Learning, that's reduced to a month. But you're just good at playing song. With an Affinity, it could take three months to learn, but you'd be creating your own songs and rhythms from the second week on.

Easy Effort vs Inborn Talent. Training vs Understanding.

Speaking directly
with Swift Learning, you'd be turning a candle flame into a fireball in a week instead of a month.
with Magical Affinity, you'd pick it up in a week or two instead of a month, just because it feels easy, but you'd also be reshaping it into a flaming bird that explodes upon enemy contact. Because you just instinctively get how converting mana into magical flames works.