Read moreCorrected Answer
MoltonMontro stated the reason the values don't add up is because the Mythical 3% chance is independent to the rest of the items (3% chance of getting a mythical, if you don't get it, then 20% chance at an Epic item, 75% chance at a Rare item, and 5% chance at a Legendary item).Short Answer
Either there is some serious issues with how the game is rounding, or there is actually something wrong with the droprates. The rare items should be approximately 5.14% (72% for the category) instead of 5.36% (75% for the category).Explanation
If you assume each item has a 1 in 22 chance, then this should be the chances:
Category Percentage Mythical 4.54545%... Legendary 18.18181%... Epic 13.63636%... ... Rare 63.63636%...
The current percentages are correct. Mythical chance is a separate factor in addition to the base rarity. 75 + 20 + 5 = 100. This is only true for old presents/boxes.
Newer presents/boxes have an equal chance for any item (100 = 100). Mythical chance is separate. Additional item chance is separate.