Combat Hit % Chance

It entirely depends on your ship attack score vs their ship defense score, the weapons being used, and the distance.
If you have a save from before that battle, it would be nice to have a look at it to see what might be going on.

You could try scanning their ship and see what its attack and defense are as well as how they are calculated. There are several different factors than can influence these values. There are also some weapons that are less accurate than others.

Kinetics are pretty inaccurate at range except for weapons that state otherwise like the Railgun and the Gauss-Coil Gun. If you are using Particle Accelerators or especially Wurtzite Cannons, you want to be up close and make sure you have a higher ship attack than their defense.

Beams are more accurate at range as are missiles.

There are a number of ways to increase your attack value (and % chance to hit respectively):

• Research the targeting algorithm techs in the Propulsion category of the tech tree.
  • The bonus attack % it provides is added to all your ships automatically.
  • The starting algorithm tech provides +25% ship attack. By the end, you can get up to +125% ship attack from the final expert targeting algorithms.

• Research the Battle Sensors module in the Weapons category of the tech tree.
  • You need to equip this on your ships. It is in the "Special Systems" category of the design screen once researched.
  • You gain more benefit from Battle Sensors based on the level of scanning technology you have researched in Propulsion category of the tech tree.
  • The starting scanner tech provides +25% ship attack with battle sensors. The highest level one provides +100% ship attack.

• Bring a ship leader with a high attack skill. Every point in the attack skill grants 10% to the entire fleet they are in. If you have multiple leaders in the same fleet, only the bonus from the leader with the highest attack skill is applied.

• Focus on getting a more experienced crew.
  • A 1 star crew grants a 15% ship attack bonus. A 3 star crew grants a 50% ship attack bonus.
    
  • You can build up crew experience by building ships at a location with Ship Support infrastructure, by hiring a leader with the Instructor secondary skill, or by fighting battles. There may be another method I am forgetting as well.

• Get into very close range.
  • This is especially important for weapons like the Wurtzite Cannon which are kinetics (less accurate in general) and suffer double the normal range penalty for accuracy. Note these are getting a damage buff in the next build due to give them more upside due to this downside.

Here's an example showing my chance to hit at a decent range:

 

@aReclusiveMind has covered the details. I just want to add that your enemy probably has a ship leader with a very high defense skill, exceptional crew experience, or both. Also, are you attacking a colony of the enemy? If so, they may have an airspace support facility helping with defense. Scanning your enemy to see their Ship Defense should reveal what may be contributing to its high defense.

Differences higher than 50% between your Ship Attack value and the enemy's Ship Defense value can start to produce very poor accuracy values.

If you have a save before that battle or a similar one please send it so I could have a look to see if there is any issue.

Thanks!

 

Yes, that explains it.

Could be a nice improvement, I took note of that for the future. Now, even if ships haven't moved before, one can think they can still do evasive maneuvers, less so the bigger the ship, and therefore the penalty on SD for bigger ships abstract that concept.

The formula of chance to hit is the following:

Chance to hit = 100 / (1 + 2^E)

with:
E = - (SA - SD) / 16
SA = Ship Attack
SD = Ship Defense​

The values of percent hit you got are inline with what's expected.

I understand that there may be a learning curve and that the actual calculations may be a bit opaque. There is information on accuracy, that it is governed by the difference between your Ship Attack and the enemy's Ship Defense value. With time, the idea is that the player will start to get a better understanding of how it works. However, the actual calculations of how accuracy is calculated are not shown (the formula above). This is the type of thing that I think would go well in the game's manual.

\Edit: And, by the way, the chance to hit you see is for "1" instance of the given weapon hitting. So, if you have 5 phasors in one weapon slot, each one has the given % to hit.

Thanks!