Even with all the restrictions listed here, more than one existing allocation could match your dsname request. Then, if you specified a ddname and one of the matching existing allocations is associated with that ddname, that is the allocation that dynamic allocation selects to satisfy your request.
If you did not specify a ddname, dynamic allocation selects the matching existing allocation whose in-use attribute was most recently removed. (Data sets allocated through JCL are considered to have had their in-use attributes removed at step allocation.)