You're not the only one :) I used to be confused, too.
There are many words in Vietnamese that used to be common or have a used-to-be-common meaning in the past but now people don't use them anymore. They currently appear rarely in a few compound words.
- Núc in bếp núc
- Búa in chợ búa (note that búa here doesn't mean "hammer")
- Sá in đường sá
- Sáp in sáp nhập (note that sáp here doesn't mean "wax" in beeswax, candle wax, etc. Many people write it as sát nhập -> wrong).
- Han in hỏi han
- Cộ in xe cộ
Learning about them is really interesting.