Although we do have a /24 now, but that will be put to other uses
I thought such short prefixes where only handed out temporarily for 6rd deployments and similar transition mechanisms.
We'll leave this unanswered as to our specifics, but feel free to guess away!
A quick traceroute reveals that at least 2001:470:28::/48 which is used for /64s on sto1 is routed only as a /48. Oddly the routed /64s and the tunnel endpoints are from a different /48. The two /48s are adjacent, but could not be aggregated into a /47.
So 2001:470:27::/48 and 2001:470:28::/48 are both routed to the same tunnel server, but 2001:470:26::/48 and 2001:470:29::/48 are elsewhere.
I cannot figure out what the reasoning would have been behind this.