Hit chance value is a huge number of factors all rolled into one single value.
You can know all these factors, though--they are both explained in detail in the manual, and you can use the combat log for a breakdown during firefights. For details see Interpreting the Combat Log.
Base hit chance before any modification is 60% It cannot be higher than 95% and cannot be lower than 10%.
+3%/cell if range < 6 +attacker utility bonuses +10% if attacker didn't move for the last 2 actions +3% of defender heat +10%/+30% for large/huge targets +15% for hack-linked targets +5% w/robot analysis data -1~15% if defender moved last turn, where faster = harder to hit -10% if attacker moved last action -3% of attacker heat -10%/-30% for small/tiny targets -10% if target is flying -defender utility bonuses
+utility bonuses -recoil (from other weapons) -10% while system corrupted (corruption% chance/shot)
Source of information is this post by Kyzrati May 22 2015.