Marshall Scholarship question Please let me know if there is a better place to post this.

From the document, http://www.marshallscholarship.org/applications/2020_rules_2_year_updated, I have,

"In addition if a candidate puts Oxford, Cambridge or Imperial College, King’s College London, LSE or UCL as their first choice institution none of these universities can be listed as second choice institutions."

I'm not sure how to read that. It seems from only that sentence, that it is somehow more difficult to get a Marshall at one of those institutions, and it would benefit you to choose an institution not listed there so the cohorts for each year are spread around...? But then looking at previous years this is simply not the case, http://www.marshallscholarship.org/scholars/winners_2018

Is there any benefit to applying to say Durham/Edinburgh/Southampton over Oxford/Cambridge?

Thanks in advance!
Without asking them directly, it's hard to say for certain what their intent is, but it seems like a mechanism to dissuade applicants from applying solely to those institutions. For example, they don't want everyone putting Oxford as 1st choice, and Cambridge as 2nd choice.