[BUGS] Strange influence of default_statistics_target

I have encountered a problem with querry plan building:

I'd set  default_statistics_target=700, run analyze. Postgres optimize had chosen plan with hash_join and it took ~1 min for qerry to complete.

Then i set default_statistics_target=500 and the plan was significantly changed and was using merge_join instead, complition time reduced in hundreds times, cost reduced drastically. 

Now i can't understand why more precise statistics leads to less optimized plan and what is the right way to use default_statistics_target parameter?

I attach both good and bad querry plans and the querry itself