The performance ranking is calculated by the overall ranking of all three indices: average response time, average availability and average consistency. If two sites are with the same number of overall ranking, they will be listed in the same ranking place.
( As above example, ranking for each index can be found at: http://www.gomez.com/us-retail-mobile/)
For the mentioned Us Retail - Mobile benchmark index, take the two websites of third place as an example:
| ||Response Time||Availability||Consistency||Total|
You can see that they have the same number of total ranking thus the are listed both as the third place. Due to there are two third places, the next place is listed as the fifth other than the fourth.