Fastest route? No, there is a better one… why?

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Hi! I planned a trip from BOLZANO (Italy) to FIRENZE. Using a BMW i3 120 Ah. The result is 4 hours and 42 minutes. Leaving the highway twice to charge. If I force the system to stop at Nogaredo Ovest (in the highway), also the following stop is in the highway (Secchia Ovest) and the route time is 4 hours and 31 minutes (and also 11 km shorter). Why does it happen? Thanks!