What was the threat level on the mission? 1 - 3 and like that for "Mostly Harmless" shouldn't be spawning those kinds of enemies. Assassination missions send you after a specific target and so there is a relationship there, but Massacre missions just task you to rack up a count of X many ships destroyed. When you drop into a system with a Massacre mission up, the game will spawn some SS specifically because you're carrying the mission (aka this is the pirate activity going on right now that you took the mission to do something about).
Those SS will be related to your mission threat level but also to your rank, ship, and its spec. This is where it gets really complicated. The game scales guys not just based on the mission but also the other stuff. In particular, I've noticed the kind of shield you've equipped seems to make a really big difference to the kinds of guys you see. Dropping even one level of shield on my main T9 (usually C7) for the extra tonnage has me going from Deadly Anaconda to Master FDLs. With a C5 I remember seeing Asps.
Courier missions all have Threat Ratings as well, and Trade rank is taken into account. So if you're Trade Elite, and you take a threat 8 courier, and you're in a Sidewinder, you get the deadly nastyconda. Usually it's the PA guy or rail guy. The laser guy is pretty easy. The explosive cannon guy is a real . From the perspective of a freighter, mind. Good bounties from the cannon guys though.
It's really complicated because they're using all of these inputs to do the scaling. Though I, too, think that it goes off-kilter and defies expectations too quickly though. It diminishes threat level as a tool. Maybe the lower the threat level, the less variable it should be; err on the side of caution, let it open up as threat goes up. They're also trying to preserve some variability, though. Like in the Courier missions, if you aren't in a top-tier ship, it's not a bad thing for you to see some enemies that are much stronger than you that you want to run away from. That situation is not as easily managed in Combat missions, though.
There is kind of maybe disconnect in terms of what they're doing on the back end and how people play it on the front end. Certain elements seem done by 2014 sensibilities, kind of thing. It all makes sense if everyone who has a 1000+ hours is on their way to Combat Elite and is always in a Large ship that's ready to go. On the other hand as a developer when you embrace all of these various playstyles, you're naturally going to get some rough edges.
Anyway, yeah, once you accrue hours and you get them ranks, the spawning system starts to break down a little bit.