It is easy to explain why one has better proficiency in common vocabulary in one's native language than in one's second language.
But when it comes to low frequency words, why does the native language still prevail over the second language?
The possibility to see a low frequency word in reading is equally low in both native language and second language, which means the exposure are nearly equal.
And you don't even need to use those words by yourself, which means they are both passive vocabulary in each language.
Why does it seem that we still have a better memory to remember low frequency passive vocabulary in our native language than in the second language? Even if you cannot remember the meaning, we are still likely to give a good guess on the rare word of our mother tongue.