Why are expected delivery times given as a range?

Solution 1:

I play lots of ETS.

The delivery range provides information on the earliest time, given optimal conditions, that you would be able to deliver the cargo. The latter time, as you pointed out, was the deadline, where arriving afterwards incurs a penalty.

I have not noticed any particular bonus for delivering earlier - other than not being late. I think the first time is really a guide as to when you can reasonably expect to arrive given perfect conditions.

Solution 2:

The earliest time is a rough estimation of your arrival time, keeping in mind resting, boat/train travel time, and road travel time at the allowed speeds (although I've noticed it doesn't calculate it very well for roads on which you can drive 90km/h.)

It does not keep in mind that you can rest on boats/trains and therefore it is quite easy to deliver almost a whole day early. There is also no problem with doing so, but no reward other than being done earlier (in-game time) is given either.