So, I don't get how sometime acceleration is more or less instantaneous e.g. you leave an USS and you're at 200+C immediately. Whereas when trying to interdict someone your speed creeps up at 0.1C increments when at 30C, for example. Both the examples happen in deep space nowhere near a gravity well. What gives? Is it just an inconsistency in the game?
Broadly, your normal [1] top speed is dependent on your distance to the most significant gravity well - which might well be 10,000Ls away, but still counts. Acceleration up to this top speed is fast.
However, as you move, your position relative to the gravity wells changes, and therefore your top speed also changes.
So, let's say you've just exited a USS in deep space, are facing away from the system's only star, and see a ship ahead of you which you want to interdict.
You start off accelerating very quickly from 0 to the local top speed of 30C. You're now moving away from the star, and the local top speed is slowly increasing. At this sort of distance from the star, local top speed increases relatively slowly, so you then go from 30C to 35C as you chase the ship you're pursuing to try to get into interdiction range, because the local top speed is now 35C. If you suddenly put on the brakes, you'll drop to zero quickly, and then be able to get back up to 35C again quickly.
If you look at the speed gauge in supercruise it has two parts.
1) The bit around the right edge. This is on a logarithmic scale, with 1C half-way up. This shows the local top speed and will only change slowly as you move closer to or further away from gravity wells.
2) The bit in the middle. This runs on a linear scale, with 0 at the bottom, and your local top speed marked. So if you're deep in a gravity well and getting {Slow Down}, you can be off the top of this scale at well under 1C ... whereas if you're in deep space you can be going over 500C and still not at the top.
Basically, acceleration on scale 2 is quick, acceleration on scale 1 is slow. If you've been flying at full throttle for more than 10 seconds or so, you'll usually be accelerating on scale 1.
[1] If you're approaching a gravity well, you can be in a situation where your actual speed exceeds the normal top speed - you will be travelling faster than you could accelerate from zero from in this position. This can be used with care to get to places much faster than you normally could.