... why a browser will show decreased rather than increased saturation given the exact same photo as was the case here...
You keep claiming it, but unless you tell us what color space is used for PS and web photos, the claim doesn't have much, if any, validity.
One example when an image is going to be significantly different, duller, less saturated (in color-managed browsers) is if you do
not convert to sRGB and do
not embed a profile. The image on the left is how it looks like in LR, PS, and the web (after conversion to sRGB), while the one on the right is export to web
without converting it from ProPhoto RGB,
nor embedding ProPhoto profile.