Same sized images displaying at different widths

I have several images galleries on my prophoto website, but one in particular is driving me mad!

http://www.victoriaphippsphotography.co.uk/emily-mark-married/

There are two sizes of image: 820×547 pixels and 820×556 pixels and on all other slideshows (on all other pages) they display at the same width. However, this particular slideshow displays the diptych images as narrower than the single images and I have no idea why.

I’ve double and triple checked all the sizes. I’ve tried deleting and re-uploading all the images several times, but I haven’t been able to figure out what’s going wrong.

Can anyone shed any light on this?

Thanks in advance…

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