It is a compromise to an alternative behavior of restarting over again from the root, which causes many non-termination problems and unpredictable behavior.
two alternatives still being considered as options at the command line are: to resume from the leftmost child of the node matching the primary node specifier (which usually is the primary placeholder, though not always); and also resuming from the node which would be the next, in a preorder left-to-right traversal, after the whole subtree substituted at primary placeholder was visited.