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No, I think this is reflecting the fact that the optimal solution for some initial states is shorter than for others, though I could be mistaken.

God's number is 20: http://www.cube20.org/

From potentially incorrect memory, one can find the actual optimal solution pretty quickly, but that needs a giant lookup table that functions like an end-game table. Their lab might not have a machine with enough memory. Keep in mind that it would be a net loss if the solver took even 15ms longer.

Or 26 in the quarter turn metric http://www.cube20.org/qtm/ which is probably what matters for a robot, since 180 degree turns take longer than 90 degree turns?

Variable disorderliness.

I'd be interest to know if the previous move affects the speed of the next.

It might be possible the longer solutions are mechanically quicker.