restore or downgrade to 9.3.1, how is this possible?

Hi I have an iPad mini 4 with original firmware 9.3.1, never updated.
Today I was trying to update to ios 10.3.5 with iTunes, but it said I didn’t have enough space, so I just erased everything and set up as new, thinking I would update and then restore from backup. However, I noticed, after being restarted and set as new, the ipad was in the process of downloading ios 10.2.1 OTA automatically, and out of curiosity, I deleted this partial update file. So the updating was stopped and the ipad remained at 9.3.1 like nothing has happened.
I am aware that the ios 9 signing had stopped last October, so I’m wondering how is this possible? If I can erase everything and always restore to factory status (9.3.1) I don’t have to worry about not being able to downgrade if I don’t like ios 10? please help me understand this, thanks!