that has to do with the ETA board itself, not the trains... cos
i think the ETA board receives signals from situated points in the
tunnels when the train passes these points.. (if i'm not
wrong)
also, the NS trains, like all others are on auto on off-peak but
manual during peak.. so i think off-peak should be more
puntual...
and during peak, since the trains are on manual, if a train is
delayed, the whole setting is thrown off easily..