Robin,
Unless the Mauser that was the Match 54s parent used a fine thread, my guess is that Anachutz wanted more thread engagement in their round action. The Mauser had a square base, perhaps less deep than a Finnish Lion but on that order, but Anschutz changed this to round, possibly to reduce machining with round bar stock. The bolt is offset in the receiver* to give the bolts more seating depth, so a fine thread would reinforce this.
*Also necessitating the lugs be spaced at 120° rather than 180° (a feature Walther retained), to ensure enough depth for the left hand recess.