Where? For example Rouge Hill is every 25-30 minutes from 7AM onwards.
You can't just arbitrarily pick a random station and declare that the service frequency. You have to look at the line as a whole, and the key stations that have the most service.
Pickering and Union on the line are the key stations where all trains operate from.
Pickerings morning run is:
06:46 07:06 07:21 07:23 07:36 07:49 07:55 08:05 08:14 08:25
And arriving at Union at:
07:38 07:50 08:08 08:05 08:18 08:42 08:33 08:43 09:10
Which means on the line during rush hour there are services available that are far better than every 30 minutes.
Not every station has the same service level and thats why we use the term "at least xx minutes" when dictating service levels because often times service is much better than that. Its the bare minimum.
With GO RER, we will have service of "at least" every 15 minutes, and at some stations, up to every 3 minutes during peak times.