When you record with AVCHD you can only record 16:9 ratio
This means when filming with a 2x adapter the 1920 becomes 3840
so its 3840/1080 --> "3,55x ratio"
The original adapter was intended for 35mm or 16mm film with a ratio of 4:3
so the ratio became 8:3 --> "2,66x ratio" with is more common in cinema.
Because the avchd does not allow 4:3 recording with the GH2, 3,55x is the only option