That's Najas for you
This tends to happen more often with them because of their super low starting Int/Mana. Prior to the slot jumping fix, Najas like this would normally jump to 3 slots upon training; however pets no longer re-calculate their control slot upon starting training so you are good with this Naja. He will stay 2 slot until you start spending training points.
I actually recently went into a more in-depth explanation about this, which you can read
here if you are curious.