I have encountered either a bug or an issue where when specifying the Y level coordinates for example 

873 30 343 in the nether

and when generating structures using the /place command the structures generated ignore the specified Y level parameter, this is easily replicable. (Also, some structures fail to generate when using this command) such structures such as mineshafts appear to select a pre-determined Y level regardless of which Y level has been specified . in my case it was 30 and it decided to generate on the nether roof.