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Corrected 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%... |
Unfortunately, those chances don't correlate with what Unturned says at all, and that's because certain categories have their own chances.
| Category | Individual item chance | Category chance |
|---|---|---|
| Mythical | 3.0% | 3.0% |
| Legendary | 1.25% | 5.0% |
| Epic | 6.67% | 20.01% |
| Rare | 5.36% | 75.04% |
However, these values don't add up either, they come to about 103.05%.
I'm going to assume that given the strange number of 5.36%, that is the number that is off. So let's reverse engineer it based on the other values, and speculate that 6.67% is actually 1 / 15 rounded.
| Category | Category chance |
|---|---|
| Mythical | 3 / 100 |
| Legendary | 5 / 100 |
| Epic | 20 / 100 |
| Rare | 72 / 100 |
There was 72 left over out of 100, and as such, it was found that the value Rare should have per category is 72 / 100, or 72%. This also lines up with the original drop chance being a total of 103%.
This also means that per item, the Rare chance should actually be ~5.14% per item, not 5.36% like Unturned claims.