Why is ForeFlight telling me to reduce my altitude to a lower flight level when using Performance?

ForeFlight checks for cruise capability for a given altitude.  Although your aircraft might be able to climb to a higher altitude, the combination of weight, temperature, and altitude may prevent your aircraft from generating enough thrust to accelerate from the slower climb speed to the faster cruise speed. In such cases ForeFlight will suggest a lower altitude where the desired cruise speed can be attained relatively quickly after level-off. 

In the event that your aircraft can reach the desired cruise altitude after burning some amount of fuel, ForeFlight will utilize a Step Climb. Step Climb indicators are shown in Route Advisor, Altitude Advisor, and the flight plan filing form when a step climb is required to reach a flight’s planned final cruise altitude. To see where in the flight the step takes place, look in the Navlog’s altitude column (requires Performance Plus or Business Performance subscription).