I'm missing 2 bytes (GRE implementation)

I'm using a GRE IPv4 tunnel between a cisco and linux machines I did some packet capture, and saw that my MTU was 1418, but the cisco was sending TCP packet with a MSS of 1380. This created a bunch of issues. When I told the cisco box to use a MSS of 1378 everything starting to work fine. So why Cisco is off by 2 Bytes? Does the GRE implementation on Linux uses 2 extra bytes compared to Cisco (or vice versa)?