In this commit the label are swap but not the descriptions:
https://github.com/jenkinsci/valgrind-plugin/commit/5ac5cf4322b13106c56dbabae021912c892f8efc

From the Valgrind manual:
"Definitely lost". This means that no pointer to the block can be found. The block is classified as "lost", because the programmer could not possibly have freed it at program exit, since no pointer to it exists.
"Possibly lost". This means that a chain of one or more pointers to the block has been found, but at least one of the pointers is an interior-pointer.