Think of it like this:
Your schedule for B1 is to overfly it at 10:00, and B2 at 11:00. You are right now at a distance exactly between B1 and B2 and the clock is 10:30. The time-deviation in the CK-panel will show 0 because you are "on schedule". BUT, the distance between B1 and B2 is 1000km which means your planned speed is 1000km/h but you are only flying 500 km/h, and this means you will arrive at B2 15 minutes late. So the fin on the HUD will be low indicating that you should speed-up. If both the fin and the ck-panel shows no time-devation you are both "on schedule" AND at pre-planned speed. Hope that explains it a bit! I believe the explanation in the manual could maybe need some expansion.