[FFmpeg-user] Why is my mp4 file larger when I do an encode with -threads 0

[Arnie Bearak]

I did an encode of a set of pgm files with the following command

ffmpeg -r 1 -b  -I  frame_%05d.pgm -b 18000 -vcodec libx264 -vpre slow Arn.mp4  
it generated an pm4 file of about 56K

But when I repeated the test  and did

ffmpeg -r 1 -b  -I  frame_%05d.pgm -b 18000 - threads 0 -vcodec libx264 -vpre 
slow Arn.mp4
it generated a file of about 102K bytes.

Can anyone tell me why the generated file is larger when using -threads 0?

Thanks