Pigs (without swords) spawn pigmen with swords when struck by lightning

I was walking around in a plains biome during a thunder storm when a pig got struck by lightning and turned into a pigman with a sword. Where did the sword come from?

What I expected to happen was...:
When the pig got hit by the lightning it should turn into a pigman without sword.

What actually happened was...:
The pigman that spawned had a sword.

Steps to Reproduce:

1. Spawn a pig and stand next to it
2. /summon minecraft:lightning_bolt
3. See a pigman with sword

Besides the logical aspect mentioned above (where should the sword come from?) it also isn't logical seen from a game developer perspective.
Pigman which spawned due to lightning are rare, right? But if they have a sword there is no difference to the ones that come out of the nether.

If they spawn without a sword it's something special to have a pigman which is not holding a sword because you can see that it was a pig which got struck by lightning.

On the screenshot you see pigs without swords and a pigman with sword spawned form a pig.