Hi,
I’ve recorded a video of the problem (just happened recently again).
https://www.dropbox.com/s/vj7q1v99cbyye23/wrong_slider_size.avi?dl=1
As you can see, all picture have correct sizes. I have run the thumbnail tool twice. The problem is the extra white space which happens occasionally.
Skipping the child theme didn’t help before, but I haven’t tested it for a while.
My Browser is IE 11, with all latest updates. I have also run into the problem with Chrome.
I will check your demo and see if I can get same problem there.