It's voltage for Pi 3. They forgot to account for voltage drop due to protection or something. Official as well as third-party Pi 3 specific adapters are rated at 5.1V to mitigate that problem.
Early alexa devices also have power supplies that are rated at 5.2V, those could be an alternative. I also had a good experience with a 30W oneplus warp charger after some power issues with cheap 5V2A ones.