From the nanog thread it seemed like the IP allocations for the IPv6 transition space (4.10) was the only space using this manual Excel process. That's probably how they initially started managing these allocations with the intention to build it into their automated systems but hadn't gotten around to it. And it sounds like they're prioritizing that work now, and have implemented an additional lay of checks in the mean time.

This is a really big egg on face moment for ARIN, but it sounds like they are responding appropriately.