зависит от версии андроида, сначала было многопотоков, потом переделали на один, все равно AsyncTask давно никто не использует
ладно, нашел
Order of execution
When first introduced, AsyncTasks were executed serially on a single background thread. Starting with Build.VERSION_CODES.DONUT, this was changed to a pool of threads allowing multiple tasks to operate in parallel. Starting with Build.VERSION_CODES.HONEYCOMB, tasks are executed on a single thread to avoid common application errors caused by parallel execution.
If you truly want parallel execution, you can invoke executeOnExecutor(java.util.concurrent.Executor, java.lang.Object[]) with THREAD_POOL_EXECUTOR.