An MTU is in use other than 1500 bytes, and corresponds to a well-known standard.
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MTU, or maximum transmission unit, is also known as link MTU. Measured in bytes, it's the maximum packet size that can be sent in one piece over a hop without being fragmented. It differs from path MTU (PMTU) which is the minimum MTU of the hops in a path. In the presence of packet loss the path MTU discovery process can generate an underestimate for the MTU. One way to verify the discovered MTU is to re-run the test a couple times to see if the result is consistent. That said, the fact that the discovered MTU is associated with a known standard suggests that it is not an accidental underestimate.
Possible secondary messages
576 bytes corresponds to ISDN
1006 bytes corresponds to SLIP
1476 bytes corresponds to a GRE Tunnel or other encapsulation mechanism