Inaccurate milling - what could it be?

I went to make what should have been a 29.534mm hex bolt, however it came out very off spec. I've no real idea why. 

• what it was supposed to be - imgur.com/a/6chSepJ
• how it came out - imgur.com/a/6r5prsT
  • note the 'bump'
  • didn't come out to dimension
• video link - 
  • it had three points holding it, and i did two that both came out the same, which makes me believe it is the cnc config not work holding
• the configs -  github.com/vetsin/linuxcnc-pm30/tree/scr...d-probe/configs/PM30

I did go through the process of checking backlash and ensuring my velocity was alright a long time ago -- but honestly it's been a very slow project and I forget what I did. Any ideas what the reason for the problem could be? What should I check first?
 

You might also want to mention what tools/holders, speeds, and feeds you're using, as well as how far off the dimensions are.

That said, my first reaction to the above is deflection- the gouge looks like where the tool ended the cut, possibly dwelled there a bit before/during retract, giving it more time to cut. If it is deflection, there's a good chance you'll feel ridges if you run a fingernail along the floor of the machined surface perpendicular to the direction of cut.

Adding a finishing cut might be all you need- I generally leave 0.1~0.2mm of stock during roughing operations for the finishing cut(s).

Generally increasing rigidity should help as well (shorter tools/toolholders/stickout, bigger tools, better worktholding, working closer to the table), but I've always thought the columns on benchtop mills look too small, so you may be limited in options if that's the root cause. The good news is that so long as it cuts OK, taking a light finish cut should still get you on dimension.