Winterdune has indeed got to the nub of it, there's clearly been a mix up.
The bird being queried in the pic is not the bird originally assumed to be a HM. The bird in the pic is one of the birds originally ID'd as a Common Swift. Apart from the fact that HMs have white underparts (and the bird in the pic doesn't), and very different ways of flying around, there's the exif data - the "comparison" pic of the Common Swift, and the mystery pic, as well as being consecutive (as mentioned by Jeff), were taken at exactly the same time - 3:04 pm.