I arrived at Heathrow at 05:00 yesterday morning and had to transfer from T3 to T5. I was ahead of the pack and as I got to the bus stop, the bus pulled away with 3 pax on board, just as the 50 or 60 people who wanted to transfer were coming down the escalators behind me. We then all stood about for 20 minutes, despite the service supposedly running every seven minutes.
This is typical of outsourced services (such as the Heathrow Transfer bus) nowadays, where the only measure is whether the bus leaves at the appointed moment, say at 05.10. Whether there are a large number of passengers approaching is completely by-the-by to those from Heathrow who gave out the contract. They will of course never be around at that time of morning to see the doors slammed in the face of those approaching, and probably can't even understand why people are dissatisfied with this approach.
The "every 7 minutes" may be a generic statement, but will doubtless only apply to some other times of the day, not at 05.10 on a weekend morning when the desire to avoid night payments will be paramount, so the schedule at that time is doubtless one bus only. The normal excuse why the full agreed service timetable is not shown is "to mke it easy to understand".