because during the compression stroke if water was present it could have just waterlogged the plug on that cylinder and you cant tell wich if you dont pull em all and also will rule out a bent rod by compression testing. you can hydrolock a motor with out killing the engine and with out breaking a rod. depends on rpm at the time. thats how i got my b20, a guy bent a rod,after pulling head the piston liked 1/4 inch from coming top dead center.