The 38400 could be close to the maximum of the SoftwareSerial library.
The codes you read seems like a baudrate mismatch. If you are sure that the baudrate is set properly, it might have to do with optimazations or different versions of the Arduino IDE.
If you have Arduino installed from the repositories, you have an older version.
The behaviour in Windows and Linux should be the same if the same version of the Arduino IDE was used.