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The behavior you observe is due to the floor division // having a higher precedence than the subtraction operator - but a lower precedence than the negative operator -, see the table of operator precedence. Compare:

sage: a = 8^2 - 3^2; a
55
sage: b = 6^2 - 9*8; b
-36
sage: n(a/b)
-1.52777777777778
sage: a//b
-2
sage: 0 - a//b  # subtraction operator
2
sage: - a//b  # negative operator
1
sage: 0 + (- a//b)  # different from 0 - a//b  
1