Don't know for sure but my guess is they use different computer models and end up with a different scale.  Whatever,  these are the same bunch of people that helped to cause the last financial crisis by giving dodgy (sub-prime) mortgage derivatives AAA status.

Their ratings are supposed to be an assessment of how likely the creditor (UKGovt) is of being able to repay the loan.  They see everything in terms of risk.  They assess that leaving the EU makes the risk of default greater so our credit rating goes down.

If they had been around back then they would have said that the invention of the wheel would cause instability and down-graded the kings credit rating.

The pound is said to have slumped, crashed, plumeted etc, etc.  It looks to me like its value against the dollar has dropped a few cents.  But that doesn't make a good headline.

The same with stock markets taking fright.  "Billions wiped off share values as markets crash" but it doesnt mention that the total value of the market is in trillions.  "Barclays Bank shares suspended as value plummets by x%" sounds catastrophic, in reality they fell by pennies

Markets always over react to any news, good or bad.  A good dollop of hysteria and panic sets in triggering a wave of selling, who benefits?  The super rich who quietly buy up more of the world on the cheap.