How to create bootable Mavericks external USB hard drive from Sierra?
You are correct that your model MBP did originally ship with Mac OS X 10.9.4.

However, in October 2015 Apple released a firmware update for the EFI Boot ROM of this model. My guess is you updated the firmware (it was an extremely important update to address a security risk). If so, your MBP is unable to boot into any version of OS X prior to 10.9.5.

You can read more a brief description of the MBP111.0138.B16 (2015-002) firmware update or more detailed info here.