To make a parallel to actual human language: you can understand well a foreign language and not be able to speak it at the same level.

I found myself in that situation with both foreign languages and with programming languages / frameworks - understanding is much easier than creating something good. You can of course revert to a poorer vocabulary / simpler constructions (in both cases), but an "expert" speaker/writer will get a better result. For many cases the delta can be ignored, for some cases it matters.