problems with Cisco Asa5505

This check works great on most of my devices, but now iam testing it on a Cisco ASA5505 and there is a problem:

On full interface saturation the traffic is shown around 200% of the interface. I am downloading a testfile with ~5Mb, but the check reports ~10Mb.

Its a 50/10MBit wan-interface and this is my check:
check_iftraffic64.pl # -H 192.168.123.123 -C public -i “Adaptive Security Appliance ‘outside’ interface” -I 50 -O 10 -u m