from the manual:
Physics actions: Global settings
These actions affect Physics behaviors in general, not just the one it was set for.
Set stepping iterations
Set the number of velocity iterations and position iterations used in the physics engine. The default is 8 and 3 respectively. Lower values run faster but are less accurate, and higher values can reduce performance but provide a more realistic simulation.
so if you set it on start of layout for 1 object it is set for all objects and will not be changed untill you set it again..