<Carsten> is a northern German name and conforms more to Low Saxon phonology than to Standard German phonology. Traditional Low Saxon does not shift the clusters /st/ and /sp/ to [St] and [Sp]. The Low Saxon word for “stone” is <steen> [stEIn] whereas Standard German <Stein> is [Staen].
From: Mark Reed
"Sorry to belabor a point that has so little bearing on your actual question. But if it was at one time Car-sten, what prevented the [s] from becoming [S] in accordance with the usual initial-cluster phonology?"