I'm not sure which website you are checking but think you will find you have it around the wrong way according to qantas.com
- Monday to Friday 26th -30th June - there are 2 * 737 (arvo, evening) and 1 *717 (morning)
- Sat 1st July only 1 * 717
- Sun 2nd there are 3 *737 and no 717.
By my count that is 13 * 737 and only 6 *717
13-14 Aug there is QF993 (so 2 * 737 that week), agree for remainder of that week only Qantas link. 20 Aug and 27 Aug there is 737. Im going to guess QF will be flying world cup players between cities in AU and NZ and likely diverted the 737s in August for that.
Also probably a seasonal thing given Tassie wouldn't be particularly popular place in Winter for tourists but the June schedule of 2 * 737s a day was what I was seeing in early December and in March when I flew. Looks like the twice daily 737s resume in October. So it appears Aug & Sept are the exceptions.