Hello!
Yes, it's really quite strange. Researching further, I saw that this question had already been answered on Tibia forum by a CM, and he said he also didn't know how to explain it.
Taking into account that bestiaries are defined in categories automatically based on exp, life and also damage, it is possible to deduce that dawnfire asura is probably stronger than midnight asura, and it is possible to check this if you have the patience to keep tanking 1 of each for some time and then analyze the input analyzer
Finally, in my opinion, it's just a bug, and because they are two creatures in the same spawn, with a very similar appearance rate, they didn't bother to fix it because it didn't make much of a difference.