Photoshop :: Photos Different Between CC And LR 5

I think this problem has persisted for a while (dating back to at least CS6/LR4).  Photos look different when opened in PS and when opened in LR.
 
I have a folder of jpg versions of images that I have my Mac use as rotating backgrounds on my desktops.  These jpg files were created from the original files without converting them from their default color spaces, so the jpgs are still in that color space.  The original files are 16-bit ProPhoto RGB files at very high resolutions.  The images were downsized, then saved as jpgs... that's it.  What I noticed was some weird blockiness (like jpg artifacts) in the smooth regions of one of my backgrounds when it came up on my desktop.  I opened the jpg in CC and sure enough there were the blocks.  I thought it might be a color space thing, and I remembered that I had converted all my wallpaper files to sRGB versions using LR (converted from the ProPhotoRGB jpgs, not the original files).  Opening one of those converted files in Photoshop CC showed no blockiness.  That made no sense to me since the original ProPhotoRGB jpg that created the new sRGB jpg (via Lightroom) did seem to have those blocks.  So I opened up Lightroom and looked at the original ProPhotoRGB jpg.  This is where I'm baffled by the whole thing.  How can the file show blockiness when viewed via the OS, and when viewed via CC, but not when viewed via Lightroom?  Furthermore, how can lightroom take a file that has these blocks in it (as shown by CC and OS X) and display it without blocks, and create a block free version of it in another color space?
 
Just for the hell of it, I went back to CC and the original high res ProPhoto PSD image, converted to sRGB before resizing and saving as a jpg, and sure enough the resulting image has no blockiness.  I can buy that... something in the processing pipeline in CC works when you manually do the conversion, but is automatically taken care of in Lightroom, but why is the same file being displayed oddly in CC (with the blocks), when Lightroom doesn't show the problem (and can create a new version of the file without the problem that opens just fine in CC)?
 
Here's a 200% screen shot of what photoshop shows when it opens the ProPhoto jpg file (the blocks are pretty evident in the blue of the sky): Here is the same exact file opened and displayed (at 200%) in Lightroom