PHP's versioning has generally been a bit weird since the abortive PHP 6 project. PHP 6 was meant to be the version that implemented full unicode support, but this proved to be very difficult, with the result that major new features had to go into the 5.x branch while 6.x remained in development (which, I guess, it still is).
5.3 added closures and 5.4 added traits, so adding coroutines in 5.5 is not inconsistent with past behaviour. I guess it's sort-of like how every version of OSX is 10.x, even though there have been massive changes between 10.0 and 10.8; it makes more sense to think of 10.8 as being 'OSX version 8'. Likewise 'PHP5' has been around for so long that nobody really looks at the major version number any more.