[nkvoll]

This likely has a lot to do with how they do detection.

Swift uses the .swift extension, which is pretty unique, so detection is as simple as checking for the file extension with a very low risk of a bad classification.

Some of the older, more popular languages uses more common file-suffixes, which may be shared between multiple programming languages, which has a negative impact on classification accuracy.

[GuiA]

Yes. In support of this theory: Github does not syntax color Swift files yet (eg. https://github.com/austinzheng/swift-2048/blob/master/swift-...)