What Causes GRIN Block Difficulty Changes?

I have been monitoring the block difficulties for the last few weeks and can see a fair bit of fluctuation, but I cannot see what causes a change in the difficulty.

Can someone help me understand this? What factors create the change in difficulty? Is there a formula that governs this?

It changes according to how long the last 60 blocks took to be solved:

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Brilliant! Thanks John - that definitely answers the question.