As for the wideband spike to lean mixtures when the throttle is closed, there is a fuel cut when the throttle is closed at higher rpm in most fuel injection systems, so the mixture goes lean when the throttle is closed. The system re-introduces fuel as the rpm comes down to idle. This stops the popping in the exhaust on over-run, saves a bit of fuel and allows the engine rpm to come down more quickly as there is no combustion.