Why can some “always successful” moves be resisted and others can’t?

The "check Sequence"

• Does it hit?
  
• will it proc (the stated % value in the description)
  
• Potency/tenacity check (chance to resist will never be less than 15%)
  

first one is pretty easy.
second one throws a lot of players for a loop, but simple enough to explain.
third one is a huge pain. but is bypassed by "cannot be resisted" in description. (tenacity down and dispell do not go through this check) ships have hard values so there is a lot less you can change here. increasing potency can help in most cases.