CheekyVisual
I would think that seasonality isn't really the issue but instead the deal which makes off-peak times more attractive. I'm sure it won't be co-incidence that up to 6 or 7 Ryanair flights leave between 8 and 9 in the evening from Bristol through 2013 with periods of 5 or so hours with nothing.It's clearly an off-peak period that's most attractive.
All I think is that Szczecin is a gap filler so that the Bristol based aircraft is doing something as I don't think Ryanair would want to put a very highly perfoming route at a peak time on a Bristol aircraft when they can do better flying it at an off-peak time from foreign aircraft and since most of Ryanair's destinations from Bristol are bases themselves, they have a lot of services they can do that with.