The simple fact is that the player doesn't need to know what the character knows. It does make better RP, but it's hardly necessary, *and* everyone can learn (maybe the basics, maybe slowly) the 'vocabulary' of any RP task (social, combat, whatever). Once you discount metagaming, all RP tasks are the same kind of thing. Things like socializing and planning are indeed trickier, but not categorically different. They all take imagination, potentially outside of your experience, potentially outside of your previous imaginings.
I'm going to try and translate.
Using a gun, in Shadowrun, is easy. It really doesn't matter what kind of gun you choose because they've all been dumbed down beyond belief and the first thing a player does is limit his choices anyway by only purchasing a small set of guns and ammo. Even the most serious of street sams only needs 3 types of ammo anyway, Lethal, non-lethal, and armor piecing. (And that last is definitely optional).
Using social skills, in Shadowrun, can be much more difficult. A socially unaware player may not even know all the options available to them. The repercussions for choosing a bad option may be confusing to them. (The repercussions for a bad choice in combat is pretty straightforward.)
Infiltration is a good one to compare it to, as player knowledge in infiltration is a definite plus. A player who knows nothing about infiltration in an urban environment at a table with people who do understand the challenges is at a severe disadvantage. They don't even know what they don't know.
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Edit: Part 2
Now at your table, and at many tables this may not be an issue. But at a table with people who use the words "roll play" to look down on others who aren't adding enough "flavor" it stands a large chance of being an issue. These people tend care about the representation of social skills and personality and may look down on any player who wants to substitute dice for a lack of knowledge.
I could, perhaps put it this way. For skills such as social and infiltration, there are TWO sets of rules. The things in the book and the things you know about from life, training, experience, TV shows, etc. and the action at the table reflects the intersection of those rules. For combat, there is no such intersection because the rules are blatantly unrealistic. Shadowrun is to combat what Kendo is to Bushido and Fencing is to Swashbuckling.