There are 3 marble in the world, and there are two civs: Dutch and Zulu
Dutch and Zulu players both improve 1 source of marble. They both have 33% of all the marble (1/3 and 1/3).
- The Dutch build an EIC in a city with Marble. They now have 66% Marble (2/3).
- There are technically 4 marble in play, but still only 3 marble tiles.
- The zulu build and EIC in a city with Marble. They now have 66% Marble (2/3).
- There are technically 5 marble in play, but still only 3 marble tiles.
- The Zulu now have 66% Marble, same as the Dutch, but because they built their EIC second, the Dutch keep the Marble monopoly. The zulu have to get a 3rd marble to beat the Dutch monopoly
This is why you fairly often get games where a civ has 112+% of their monopoly resource. They built an EIC in a city and already control all the resource tiles for that luxury, so they have more of that resource in their control than what is on the map, hence they have >100% of the resource.