IXXAT interface send_periodic() unexpected when period above 0.5s

When using the command "bus.send_periodic(msg, 0.50)" with the IXXAT interface, the message is send with the defined period up to a period of 0.5 seconds but anything above that (e.g. 0.6) results in a value of about 0.6 - (n*0.5) sec.
So period of 0.6 = 0.1; 1 = 0.5; 1.1 = 0.1 sec. etc.

I don't see this behavior with other interfaces (e.g. vector, kvaser)