I know. o7
I've heard several newbies ask, 'why does it say slow down? I can't!' so on the intuitive level, it's a bit confusing.
Understanding how it goes, basically on the Gravity Well level, perhaps by (two) examples;
1. You're in SC heading to a station base when a USS pops up. Targetting it, the 'automated' element of your FSD is suddenly trying to decellerate you quicker than it's able to. You lose control of the throttle because your engines can only cool down (ability to decellerate) or warm up (ability to accelerate) at a certain rate, and your ENGINES end up in a 'condition' of overspeed.
2. Likewise near planets but where it's now (still a description of your engines) to say that your FSD "loses traction" when you're in influence of a gravity well. Either you're coming in hot and trying to decellerate (but the planet gravity prevents the collapse of your FSD warp) or because the engines are ramping up the delivery of power to accelerate but you lose of traction now trying by to create a warp inside a gravity well means under acceleration you're burning out 'the clutch' (for want of a better expression) your engines are providing more power than can be transmitted to the road at this time.
(Lastly, for me the "logical disconnect" is as if 'slow down' is being 'sent' by the planet, telling you to slow down. On ship, it would say something like 'you are being slowed down" or 'slowing down' but those are both longer, a bit less sci-fi' maybe. Overspeed (or something like it) is both reasonably snappy and for me describes the "run-away condition" of the engines/warp when you get the alert)