Assuming each step is required to fail before the next step then yes those at the final steps will have a lower chance to be triggered.
If you have an item list of 10 items all at 10% drop rate assuming trev's situation as described:
Code:
Item# | % chance to get to this step
1 | 100%
2 | 90%
3 | 81%
4 | 72.9%
5 | 65.61%
6 | 59.05%
7 | 53.14%
8 | 47.63%
9 | 43.05%
10 | 38.05%
No item | 34.87%
If you're item 10 and you're only getting to even roll 38% of the time even though you have 10% chance to roll just like item one. You're only getting the opportunity less than half as much. Clearly you wont drop as much, and also despite having 10 items at 10% drop rate on the slot you would not actually ever get 100% drop unless the last one was 100%.
Of course this is with how Trev describes it, I haven't taken the time to actually look at how it's implemented.
That said if it isn't currently a single roll system for items that share probabilities it should be.