Hi sparkalina!
Seasonal rates depend on the individual hotel/hostel. It all depends. Unfortunately for us, there's no "rule of thumb" other than you can count on higher rates during their festivals like Semana Santa and Ferķa de Abril. Not sure what's going on in Seville in October but you're right, everyone's "High Season" includes October.
Here are a few examples for you:
In Seville,
Hotel Europa has varying HI/LO/MEDIUM season rates for 2006:
High: May 2006
Medium: June/July/August 2006
Medium: September 2006 - From 1st till 7
High: September/October 2006
Medium: November/December 2006
Another example in Seville is
Hotel Alminar :
Low
01/01-23/02
11/06-31/08
01/11-31/12
High
24/02-10/06
01/09-31/10