The copyright owner gets to decide how to license their stuff. So my guess, is Apache and GPLv2 together makes sense if author wants to give the user the maxim

The copyright owner gets to decide how to license their stuff. So my guess, is Apache and GPLv2 together makes sense if author wants to give the user the maximum choice in combining the library with other software. That because depending which lawyers you ask Apache and GPL are not always compatible. So say your code is Apache and you want to use RocksDB, you can pick to use the Apache license for it. Your code is GPL, then you can pick GPL.