RMS values above 0 dbFS?

I wonder why several RMS meters I'm using for music production show RMS values above 0.

For example, when using an input signal with a constant DC of 1 (0 dbFS), I get these results, although Wikipedia says that in such cases, the RMS is equal to the amplitude of the direct current.

TT Dynamic Range Meter (RMS 3.0)

Whereas other meters show the values I expect:

Voxengo SPAN (RMS: 0.0)

Why is it that meters come to such different results?

$$\int_0^{2\pi}\frac{\sin(x)^2dx}{2\pi} = \frac{1}{2}$$ $$10\log_{10}\left(\frac{1}{2}\right) \approx -3.010299956$$