Because SLR re-encodes for streaming at a certain bitrate and the original files delivered by some studios are just lower in bitrate.
For example iStripper scenes are generally lower bitrate, perhaps they are using specific encoding optimizations for the static background where only the performer is moving. You can see it in the screenshots mosiac like https://www.sexlikereal.com/virtualreality/mosaic/id/28995 it says 15.29 Mbit/s video bitrate, The SLR re-encode is probably slightly more like around 18 Mbit/s for 2700p.
It's fine to just grab the smaller original file, it's not like the re-encode adds detail. But apparently some rare cases the original file doesn't play well in deoVR/SLR app and in that case you might want to try the SLR encode instead.
I usually grab the original even if it's slightly larger as with re-encoding you always have a little loss of detail. Some originals are gigantic though with 120+ mbit/s.
