I think this might be a bug — when I used an HP potion, my Pokémon’s HP went back to 100, even though I clearly remember it being 1xx before that. I’m sure it was over 100, but after using the potion, it returned to exactly 100. I’m not sure whether this is a bug or an intended feature, since there’s no detailed information about it in the game or on the wiki.

Good Day @Huynhchau

 

From what I can deduce, it appears that you attempted to use the Pomeg berry on Cloyster or Kadabra (which failed assuming they have 0 HP EVs).  Then you successfully used a Pomeg Berry on Zapdos two times, reducing Zapdos's HP EVs by 20. Finally you used a HP UP on Zapdos to increase the HP EV by 10. Yielding in a net loss of 10 HP EV. 

 

It was likely at 110 HP EV at the beginning of the sequence then dropped to 90 EV and finally ended at 100 after using the HP UP.

 

With that being said, it does not appear that there is a bug with the information you have provided and I will now be locking this thread.

 

Thank you for bring a potential bug to our attention and if you have have any further questions or concerns do not hesitate to reach out.

 

Regards,

Bitterblade