the head is aluminium and the bolts are steel. aluminium expands about twice the rate of the steel bolts and as the engine heats up, the head will expand 100% more than the studs want to. the very reason that the std studs are 'stretch bolts' is to accommodate this situation as they have a greater stretch /force and are more elastic than normal bolts so they will return to the original length when cold.

when 6 long studs are fitted the material of the long studs (steel) will also expand at half that of the head but since they are twice as long as the head is thick, the situation becomes about equal. i don't know if arp considered this or just jumped on bandwagon of supplying bolts as there were lots of folk rebuilding cosworth engines but if their studs are just 'high quality steel' without any concessions to the above point then that's maybe why they seem to pull the threads out