That's actually about right. Buses roll out at 6:30, arrive at resorts about 6:35 to 6:40. Since buses run less frequently, the next bus will arrive about 30 minutes later - 7:10 or so. If you arrive at 6:45, you will have just missed the first bus and will have a bit of a wait for the second one.
Sorry, but this info is rapidly becoming a "Disney legend."
While it might be the percieved case on selected mornings,
(and it often gets repeated on these boards)
there is not a "specified schedule" of 6:30am buses, followed by a 30 minute "hole."
The buses roll out 6:30-ish and get dispatched by computer to the various resorts and parks. They reach their destinations and are dispatched, again and again, to the various locations according to the guest-levels (either actual, or pre-calculated.) Assuming enough buses are rolling on any given day, the "10-20 minutes between buses" average should be maintained from 6:30-ish, forward until the parks have been open for a couple of hours, at least.
In my personal experience (multiple resorts, dozens of trips, nearly always to the parks advance of rope-drop and/or an early ADR) we've always encountered multiple buses to the parks in the early mornings... rollin', rollin', rollin'.
Can there be a "hole?" You bethca! But, it will generally be to the park "YOU are headed for."
In the mean time (in that same "hole" of time) you will generally encounter multiple buses heading elsewhere.
If there was, indeed, a calculated "30 minute hole" after the first park bus runs,
guests would see NO buses at their location between, say 6:40 and 7:10am.