1. If you are comparing 240v to 208V, 240V charging will be faster than 208V. Many business electric feeds are 208V.
2. If it's hot, the AC will kick in thus eating up some of the current that would normally make its way to the battery
3. Bunches of people in the area are all using their AC, thus bringing down the available power from the grid, enough to be noticeable
Some charging stations may not supply the full 3.6kW (or 3.3kW in case of Gen 1) required for max charge rate, even if they are L2. Some of them may operate at lower voltage (208 vs 220 vs 240). External temperature (very hot or very cold) is another factor. Delayed charging based on ToU and Departure Time is yet another factor. All of these, and more, can affect charging finish time. To err on the side of caution, both the car and the app estimate more time that what turns out to be actually required. After a few months, you may notice a pattern.
In times of peak load, the free public charging stations will throttle back on supplying the electrons to vehicles connected. It has happened to me a few times during the triple digit temperatures when all buildings are using airconditioning.
One of the great engineering screw-ups in the US was not to acknowledge 3-phase power exists when they designed EV infrastructure and J1772 for the US cars. It would have been a very minor change to permit 277vac. And a small change to allow 208 3-ph and 480 3-ph.
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