Problem with offset path

Hello, 

I have a problem with the function Object > Path > Offset path. I want to recreate this image below:

To create this, offset is set to 10px. If I use these settings in my own object drawn in Illustrator, I get this result as seen below:

 I don't know what I did wrong. I tried to make the object in the X larger and also decreased the number of pixels for the function "offset path", but that didn't really helped.

Thanks in advance!