Oracle returns different results when executing a sql with bind variable or with a fix value
The results would not be guarenteed to be the same if the execution plans are different. Without an explicit sub-query ordering the rownum will be based on the order in which Oracle selected the data from the tables / joined tables. If the plans differ on how the tables are joined and in which order, the first 12 values read will be different.

To make this more deterministic, use some kind of explicit ordering on a subquery and then rownum that on the outer query. It would be normal to want the first X records based on some explicit ordering, instead of 12 random ones.