Encode Puts _1 At The End Of All Files Except Ones Made With Iphone

I have done two batches of encodes of the same files with Media Encoder.

In both instances, Media Encoder's new files were Filename_1.ext.

That is, unless the original files were from an iPhone.

One encode was h.264 and the other was DNxHD.

Is there any reason for this?

JJ