Volbeat and Illumise "Don't like each other much"?

I don't understand why Volbeat and Illumise "don't like each other very much" when they are left in the daycare? They both belong to the same Pokemon type of bug, and they both belong to the same egg groups of Bug and Human-Like. With all these factors, wouldn't the likelihood of an egg appearing be better?

Solution 1:

The likelihood of an egg is determined by three different factors:

• Species
• Egg Group
• Trainer ID

These factors result in the following likelihood for eggs (from highest to lowest):

• "The two seem to get along very well." - Same species, different Trainer ID
• "The two seem to get along." - Different species, different Trainer ID or same species, same Trainer ID
• "The two don't seem to like each other." - Different species, same Trainer ID

These resulting messages will result in an egg. However, if you get the following message:

• "The two prefer to play with other Pokémon than each other." - Different Egg Groups, same gender, or in Undiscovered Egg Group

Your Pokemon will not breed.

While Illumise eggs will occasionally hatch Volbeat, they are still counted as members of separate species. Thus, you will get the different species messages.