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I would presume createtime is the time stamp of the node's creation. Historically, files in Unix, and derived systems, had 3 time stamps: createtime, modtime and accesstime, referring to when a file was created, last modified and last accessed. Even when storing the nodes in a database, those names could be used as the corresponding field names. Also, it's quite possible for those fields to be auto-filled when a node record is created, updated or accessed.
It may be that the database engine actually stores a higher resolution value, internally, than is made available to the program code using the database's API. Therefore, the ORDER_BY modifier to the SELECT operation may be using a higher resolution time stamp value.
Also, it's possible that behavior similar to for (keys %hash) is occurring. If that is the case, then maybe changing:
would give the result you expected.
(Assuming, of course, the database engine supports that.)