Because you listen for words in your native language you recognize or are similar to help fill in the major gaps for what you don’t understand while listening for the stuff you do comprehend.
Happens to me for Tagalog, the Filipino dialect my mother speaks. Also happens to me for French because I took six years of it across middle school and high school, and sometimes Japanese from just watching subbed anime since the 90s; and I never personally had to ever really use those languages myself in order to communicate, so my sentence syntax would be horrible if I even tried.
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