ESS and problem with fping fast transmission

I've tested today something with fping program to check my internet ADSL internet connection and it have a strange behaviour:

If i ping with a time between ping less then 500 ms it seems to work bad with ess active. It seems to lost half the packet...

G:\fping217>fping www.google.it -t 400

Fast pinger version 2.17
(c) Wouter Dhondt (http://www.kwakkelflap.com)

Pinging google.navigation.opendns.com [208.69.34.230] with 32 bytes of data ever
y 400 ms:

Reply[1] from 208.69.34.230: bytes=32 time=56.6 ms TTL=55
208.69.34.230: request timed out
Reply[3] from 208.69.34.230: bytes=32 time=56.3 ms TTL=55
208.69.34.230: request timed out

Ping statistics for 208.69.34.230:
Packets: Sent = 4, Received = 2, Lost = 2 (50% loss)
Approximate round trip times in milli-seconds:
Minimum = 56.3 ms, Maximum = 56.6 ms, Average = 56.4 ms

If i ping with -t 500 all goes well.

G:\fping217>fping www.google.it -t 500

Fast pinger version 2.17
(c) Wouter Dhondt (http://www.kwakkelflap.com)

Pinging google.navigation.opendns.com [208.69.34.230] with 32 bytes of data ever
y 500 ms:

Reply[1] from 208.69.34.230: bytes=32 time=54.8 ms TTL=55
Reply[2] from 208.69.34.230: bytes=32 time=56.0 ms TTL=55
Reply[3] from 208.69.34.230: bytes=32 time=56.1 ms TTL=55
Reply[4] from 208.69.34.230: bytes=32 time=55.1 ms TTL=55

Ping statistics for 208.69.34.230:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)
Approximate round trip times in milli-seconds:
Minimum = 54.8 ms, Maximum = 56.1 ms, Average = 55.5 ms

If i disable ESS it goes well too also with -t 10 (just 10 milliseconds between pings):

Fast pinger version 2.17
(c) Wouter Dhondt (http://www.kwakkelflap.com)

Pinging google.navigation.opendns.com [208.69.34.230] with 32 bytes of data ever
y 10 ms:

Reply[1] from 208.69.34.230: bytes=32 time=55.8 ms TTL=55
Reply[2] from 208.69.34.230: bytes=32 time=54.4 ms TTL=55
Reply[3] from 208.69.34.230: bytes=32 time=57.7 ms TTL=55
Reply[4] from 208.69.34.230: bytes=32 time=55.7 ms TTL=55

Ping statistics for 208.69.34.230:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)
Approximate round trip times in milli-seconds:
Minimum = 54.4 ms, Maximum = 57.7 ms, Average = 55.9 ms

I've done the test in interactive and automatic filtering mode and it get the same problem.