That was a lie.
Arthur's mana reserve contained significant mana, as he absorbed supernatural energy during his recuperation period. After rescuing Dino, the crimson-eyed man would lose 15-20% of his current mana reserve.
The amount was still significant, but much lower than what he'd stated.
It was done simply to bait Randy into displaying his true colors. Now that he and the others were interacting more often, the crimson-eyed man couldn't risk letting down his guard for even a second in fear of betrayal.
'This is definitely paranoia…' Arthur concluded, but he couldn't allow himself to relax before confirming Randy's true nature. Only then could he come to trust him.
Even after that, he needed to maintain an attitude that was not overly familiar, as jealousy and envy were usually the crux of betrayal, and what sparked the massacre on the 98th floor down.