Each difficulty period, the difficulty cannot change by more than a factor of 4. This means the difficulty cannot increase by more than 400%, nor drop by more than 25% from one difficulty period to the next. This means that if it took five weeks to find 2016 blocks, the difficulty would only drop by 25%.
I have a strong feeling you are wrong, based on this code which enforces the difficulty adjustment limit
if (nActualTimespan < nTargetTimespan/4) nActualTimespan = nTargetTimespan/4; if (nActualTimespan > nTargetTimespan*4) nActualTimespan = nTargetTimespan*4;
It means the difficulty adjustment can be anything equals or less then 75% to the downside.
So for simplicity, a 10T difficulty can change to any number between 2.5 and 40, unless i am reading code wrong which is unlikely but i got old anyway.
You are correct, based on my reading of the relevant code, what actually happened, and the relevant wiki articles, the correct way of explaining maximum changes in the difficulty is that the difficulty can be no less than 25% of the previous difficulty, not that it can change downwards by no more than 25%.
The difference is a single word but it makes a big difference.