There is almost no difference to default morale. Some factions have plus or minus 1, but compared to other factors, it is relatively negligible. You can read the details here.

Since it sounds like your theory is based on few battles, I would rather consider a different explanation:

  There is a large variety of other factors affecting the morale of enemy units. Usually, more expensive units come with higher morale, such as a king's guard will keep fighting when a usual knight will have routed long ago. Nearby units, especially inspiring units, will increase morale, and very experienced units also have a very large bonus to morale. Most important though are the ratings or the generals involved on both sides.

There is almost no difference to default morale for the individual factions. Some have plus or minus 1, but compared to other factors, it is relatively negligible. You can read the details here. Poland does in fact have a tiny edge.

Since it sounds like your theory is based on few battles, I would rather consider a different explanation: There is a large variety of other factors affecting the morale of enemy units. Usually, more expensive units come with higher morale, such as a king's guard will keep fighting when a usual knight will have routed long ago. Nearby units, especially inspiring units, will increase morale, and very experienced units also have a very large bonus to morale. Most important though are the ratings or the generals involved on both sides.

Finally, the fact that they were defending their last capital weights in very heavily - units without a way to retreat will fight far longer. I'm not certain if they can flee at all - which would explain fighting 'til last man standing. However, I've also had cases of having to kill every single enemy in regular battles (vs AI) with ways for the enemy to retreat.