Why ETOPS 207 minutes?
ETOPS 138 and 207 are 15% extensions of ETOPS 120 and 180. 

The reason for that is that with the ETOPS 120 rule, most of the Atlantic could be covered when using Kangerlussuaq (Greenland), Gander (Newfoundland), Lajes Field (Azores) and Keflavik (Iceland) as alternative airports. Only some small triangles of the Atlantic were outside the 120 minutes flight time limit from these airports. To cover the whole Atlantic, the next step up in ETOPS (180) would be necessary. But since that is such a big difference, a 15% extension of 120 minutes, ETOPS 138, was approved which allows for covering all of the Atlantic with few extra requirements. 

For ETOPS 180 it is similar, but then for the Pacific. Normally ETOPS 180 is sufficient to cross the Pacific, but ETOPS 207 allows the 777-200ER/-200LR/-300ER to cross the Pacific ocean even when Eareckson Air Station on the western end of the Alaskan Aleutian Islands is closed.