Countries are not individuals. A country isn't a single unified entity.

There are lots of people and groups and corporations inside a country that may act independently from each other.

A company deciding to import something while a different company in the same country exports the same thing may make sense for them both, especially when the trade barriers are low.

Countries also aren't always small. Importing something in one side of a country while exporting it on the other side might make sense when that is easier and more profitable than trying to meet an internal demand by transporting the resource across the country.

There are also other factors like timing when the a few weeks after something was exported importing the same resource becomes a good idea again.

People and companies also may have contracts and commitments they made before that make them import and export stuff when it would not make sense for others in the same place and time.

All this mostly applies to free market economies. In planned economies where there is in theory one central coordinated body making all decisions many of these factors would be lessened. In practice it still happened though.