Peak Over Metering never light in 24bits record

[ppm]

When I record with the Fast Track Ultra 8r, the red clip light of input track on Protools8 never flash, even if the peak leds of FTU8r flash and I hear the overload. In fact the metering display is full range but never clip with the 0.0dbfs.

Furthermore, this problem occur only when I record in 24 bits deph on protools session, and if I record with 16 bits, the peak light flash normally.

Besides, with an other interface, the Maudio - Audiophile USB- , it's ok in 24 bits.

Can anyone else verify this and suggest an explain ?

Thank's a lot for your help.