Athene's Mana-font is bugged?

First time posting a new discussion on the new boards, and I tried using the search function to see if anyone else already addressed this....but it didn't prove too helpful. I'm linking two images, one with me at full mana and one with low mana to compare the increase the mana-font passive gives.

In the first image you can see my Mp5 is 22 when at full mana (Meaning no contribution from Mana-Font).

In the second image, you can see that my Mp5 is MUCH higher than in the first image. It's 40 Mp5 higher (totalling 62 Mp5). Mana-Font reads as "Restores 1.5% of missing mana every 5 seconds."

Meaning that Mana-Font should be contributing an additional (622-23)x.015 = 8.985 Mp5. Yet, it's contributing nearly 5 times that much. Is it possible that Mana-Font is returning the 1.5% mana every second, instead of every 5?

Or am I just misunderstanding how this all works?

Sorry again, btw, if this has already been posted and I was just unable to find it using the search function.

Edit: I was testing to see if I was correct about Morellonomicon being comparable to Athene's in mana return (aside from kill/assist portion) assuming I'm playing a champion that didn't build any additional mana. Mathematically it should be, but I found that some numbers didn't quite add up.