Indeed it does seem implausible, because that's not what the data implies. Samsung only makes 5,934 distinct Android devices if you assume all devices have equal market share.

The graph below shows that Samsung's share of the devices is made up largely by the S3. The percentage is of all Android devices based on those 11,868 models, not a percentage of the distinct models themselves.

The key word is distinct. Samsung could have made 1 model of phone, it doesn't mean that one model wasn't much more popular than any other android device. Not every model of Android device has the same sales figures.

Yes, the data is inaccurate, some of those devices are in fact ROMs (ROM is basically a custom Android build).

And a custom ROM should be treated as a different device.

In that case we should also have iOS 6, 6.0.1, 6.0.2, 6.1, 6.1.1, 6.1.2, 6.1.3, 6.1.4. Suddenly there are a lot more versions of iOS.

Also many of those ROMs might only run on a dozen devices, trimming the long tail, remove say any ROM used by < 100 people, might be a good idea.

You mean every device / ROM combination should be treated as different device? Why? Why not every device / ROM version combination?