The Collect filenames are milliseconds since the Unix epoch, Jan 1 1970. This was selected to balance length and risk of collision across all submissions to a form. Length is an issue because Windows still has a 256 char full path limit. That’s one of the reasons most exports are flat (all submission attachments at the top level rather than in subdirectories by submission id). Another natural option for automatic filenames would be uuids but those are long. Spec proposal: naming media files has more discussion around submission attachment names.
We don’t know what consideration went into the Enketo naming. Collisions across submissions to a form are likely. We’d like to change that default naming scheme but we’re not sure yet whether we’d want to use the Collect approach or design another one that both could follow.