In order to coast, the shafts which transfer engine power must be disconnected from the shafts which take that power to the wheels, otherwise the continued movement of the vehicle also keeps the engine rotating, which equates to a slight braking effect. In many vehicles, this either takes place in the torque converter (in an automatic transmission or CVT), or by disengaging gear in a manual transmission.
On vehicles with regenerative braking, some level of regenerative braking may be occuring when coasting. Normal coasting without a regenerative effect will last longer, as the energy keeping the vehicle going must be used up to generate electricity, even if it isn't a lot.