[Timo]

Delay = sound recording = memory.

The Virus has only so much temporary memory available. A few seconds of recording space.

Depending on the tempo and time-signature (the speed and therefore the amount of time that requires recording for each delay), some time-sigs are ignored if they exceed the memory (the available recording space) for the delay for any given tempo.

Or that's what I think causes it when I found the same myself (on my Indigo).

This is indicated further by slowly increasing the tempo for an ignored delay value. You'll find at some point it 'switches' to its real delay value when you've increased the tempo speed just enough to be able to fit all the delay for any given time-signature into the available memory.