Rail comparisons usually stop at the station nearest your destination, but the actual door-to-door time depends heavily on what happens after you get off the train. Paddington, for instance, might be a short walk from your hotel or a further Tube or taxi ride away depending on exactly where you are staying — and that last leg, with cases in tow, is often the part travellers underestimate when comparing the Heathrow Express headline time against a direct transfer. A door-to-door car removes that final unknown entirely, since the quoted journey time and price already account for your exact address, not just the nearest station.
The comparison also shifts with the time of day and day of the week: rail options are at their least frequent very early or late, exactly when a flight is most likely to land, whereas a pre-booked transfer runs on your schedule rather than a timetable. If your arrival time is uncertain because of a long-haul flight, that difference matters more than the headline price comparison suggests.