Is my sprint behind schedule or ahead of schedule?

[Chris de Vries]

Concider the folowing sprint reports that were taken at the same point in time. The burn up chart shows that the team is ahead of schedule (line above the guideline) while the burn down chart shows that we are behind schedule(also line above the guideline). The scrum master wants to know what is the case?

[Jack Brickey]

Well the team was doing great until May 8 when someone churned your sprint and added 15 SP. nonetheless it looks like the team is doing well at the moment. The best time to assess the sprint is at the end.

[Chris de Vries]

A developer claims that he needed to redo some work because of unclear specifications. I suspect that we are now behind schedule. That is no big deal as long as good value is delivered. 
But what I don't understand is the fact that the line in both graphs are above the guideline. 
If we are behind schedule then I would have suspected the line above the guideline in the burndown chart (as it is now) and under the guideline in the burnup. But that is not the case. I'm confused by the burn up chart.

[Chris de Vries]

So my question boils down to: Why is the line above the guide line in both the burnup and in the burndown?

Is this a bug?

[Chris de Vries]

I just noticed that the burn up starts a day earlier than the burn down? I didn't change the start date. I made the screenshots right after each other. No one else was in office so that's weird.

[Chris de Vries]

And one more thing: when I look at the charts now: the burnup chart still starts at may 4th while the burn down starts at may 5th.