What is this particular download for in the nightlies? What is this difference between this and the others?
I believe that is the 32-bit only apk.
In September, the Play Store will only allow packages that include 64-bit applications and libraries inside, so we’re having to modify our package to include both 32-bit and 64-bit applications and libs inside the same apk. If you have a 32-bit device and ROM, it will use the 32-bit stuff, but if you have a 64-bit device and ROM it will use 64-bit instead. People running 64-bit devices who still want to use a 32-bit application and libraries (for example, to take full advantage of PCSX-ReARMed’s 32-bit dynarec and NEON32 renderer) will need to sideload the 32-bit-only apk.