I think it just looks that way in this screenshot. I even went back after your post and looked at the most recent mesh of the horseshoe and its references. But there is a little roundness to it. It's not entirely leveled.
The fact, that the horseshoe currently doesn't sit tightly on its base is due to the base mesh not being defined enough. Perhaps it it is currently visually misleading.
But... the horseshoe is a headbreaker. Hardest shape on the bridge to rebuilt. Slowest progress of all.