If you supplied a flight ticket and accomendation information for 90 days, but applyed for 60 days, then there would be a discrepancy between the 60 days you are allowed to stay, and the 90 days you are proving you will be staying, since you havent been granted the 30 days extension yet.
The rejection is not vague, since you will never be told a reason for rejection, as it say, on the Embassy website.
Next time research the requirements before applying.
As always it depends on the Embassy you apply at, and how good you are selling the idea, but it is a part of Thai culture, perhaps even more than muay Thai and cooking classes.