The comparator output from a brewing stand is inconsistent between the different slots in the stand.
What I expected to happen was if the top slot in the brewing stand had a full stack of items but the three lower slots were empty, the comparator output would be about a quarter of the full fifteen - say four. This would have been consistent with the comparator output from a chest where, if a third of the slots are full of stacks of items, the output is five.
What actually happened was the comparator output was fifteen. However, if the top slot is empty but one of the lower slots has a water bottle, the output is only four, as expected.
Steps to Reproduce:
1. Place a brewing stand with an adjacent comparator and enough redstone to see the output level.
2. Put a stack of nether wart (for example) in the top slot of the brewing stand's GUI.
3. Check the output level - fifteen.
This also affected the previous snapshot.
UPDATE: Having read Dinnerbone's explanation of the algorithm in
MC-6165, the problem appears to be that the top slot is assumed to only be able to hold a single item. If four items are placed in the slot it is calculated as being 400% full, giving an average over the four slots of 100% and hence an output level of fifteen. With anything over four items in the top slot, the brewing stand is calculated as over 100% full and the output is capped at fifteen.
This may be related to
MC-2034 - the top slot does not work as expected when shift-clicking items into the brewing stand.