When the OG Droid went from Eclair to Froyo there were 2 versions available. The OTA was much smaller while the other supposedly was for the Verizon stores in case someone came in and said their OTA didn't work The larger file was the complete Froyo package, rather than just an update patch. Both installed the same way.
Yep. Unique circumstance, though - due to the fact that the standard OTA just wouldn't install on some phones. Generally the stores get the sbf/fxz files for this purpose, but in this case I believe it was to keep people from losing their data.