Yeah I'm L20 and the same thing happens around my server as well as the OP, yes there are many teams that have "Tanked" which is delibrately missed out on promotion to have another go at winning the title the following season, but there are also teams that miss out because the active number of teams per league is higher than 7, meaning that some do miss out just because they had a poor season. I've noticed this kind of thing from around L15+, not sure what its like around on my server now on the levels that are in the lower to mid teens, but I'd imagine it still happens.
I guess tanking is not too bad of a strategy if you are not able to farm any tokens and are a non-T buyer because at the end of the day, you don't exactly receive any sort of penalty for not promoting, you only end up a little further away from the servers top level and don't play in the next seasons CL that is it really.