It has to do with the time scale.
If I have mine set to 8 minutes, I don't get any of those little mini bars.
If I set it to longer, they seem to appear although I couldn't put any major logic to their spacing. I tried seeing if it correlated to any action (manual shifting, changing eco modes, etc) and none of them caused a change after watching it repeatedly on my 45 minute commute.
Think it might actually just be some awkward attempt at a subdivision graphic that wasn't implemented right - but it definitely changes with the time scale.
Current: '15 M235i Vert
Previous: '13 335i, '08 TL-S, '00 Corvette