Double dot on spotlight calculator
What is happening, is the calculator splits the numbers and dots into the following:

5..6=3 turns into 5 * .6 = 3

As we know that . is another way of saying 'multiply' its simply, 5 multiplied by .6

As for 523..6.1=31.38, it turns into 523 * (.6 * .1) = 31.38