I was able to replicate at least some of this issue in our test environment the other day. For us, these orphaned records seem to be created when a Retail statement is calculated, cleared and then subsequently deleted on a user session at the same time as the "post stock" job runs as a batch process. When I did this, transactions were reserved on InventTrans against the deleted statement ID in addition to the new statement ID.
Does this sound similar to what might be happening with you? I'd be really interested to hear either way.