I have posted up the following on the GuildOx
About page to explain the rankings:
How do the new GuildOx 25 and 10 rankings work?
GuildOx "implies" the raid mode on a kill by kill basis by counting the number of players in a guild when you completed your first kill. If it is more than 10 players then we tag it as a 25m kill, if it is 10 players or less then we tag it as a 10m kill. We then look at the latest three progression kills and if they are all done in 25m mode then the guild is eligible for the 25m rankings. If the last three kills are all in 10m mode then the guild is eligible for the 10m rankings. Note that all guilds are eligible for the combined 25+10 rankings.