This sounds like a bug. Either that info is correct and the phoenix will actually decrease your MM effect (which is probably not how it's supposed to work), or that info is incorrect, in which case the way it's displayed in game is not working correctly. Either scenario indicates a bug and I recommend reporting it.The Storm phoenix displays MM "+ 55%". I have a level 21 trade centre. MM spells display a "+87.5%" effect.
My guess would be that since an unboosted MM spell provides + 50%, the storm phoenix at level 1 provides a 10% boost to that (10% of 50 = 5) that the storm phoenix plus my level 21 trade centre would provide 10% extra of an 87.5% boost, 96.25% total.
Without placing and using pet food, all we will be able to do is guess
I suspect how it actually works (or is meant to work) is it gives a 5% increase per level over what it would otherwise be. E.g. if your normal MM spells are 50%, then with a level 1 storm they'll become 55%, with a level 10 they'll become 100%. For you, 87.5% will probably become 92.5% to 137.5%, depending on storm level.
Placing and using the phoenix will be the most definitive way to find out, but contacting support will likely work as well. If you report this as a bug you can probably find out what it will actually do if used in your city. If you do, let us know.