Once upon a time there was a biggish bird living in Europe (and other places) that was known by the English speakers as the purple gallinule, meanwhile in the US (and other places?) there was a similar looking but much smaller bird called (by the US English speakers) the purple gallinule. Confusing eh?
To avoid this confusion of English names (for which there is no standard - buzzards in US = vultures to UK, buzzards in UK = hawks in US) a committee was set up to standardise the English names internationally.
The American purple gallinule is now known as the purple gallinule, and the (European) purple gallinule is now know as the purple swamphen.
Unfortunately many people (and book authors and publishers) are reluctant to accept the new naming standard, and so the confusion continues.
How many Americans refer to their robin as the American robin, and how many brits refer to their robin as European robin? Both names are now the recommended international English names.