Difficulty Score is the EWMA (exponentially weighted moving average) of XSS/hour.
Rare but possible to exceed 200 but it can also mean the fitness signature is off.
You can determine this by reviewing the activity chart and, if necessary, tweak signature values on Advanced MPA tab to generate more reasonable results.
See this thread – Difficulty Score - did I break it? - General - Xert Community Forum (xertonline.com)
Reference –
Difficulty Score – Xert (baronbiosys.com)