Hmmm SQ might be restricting suites availability on multiple routes that they plan to launch the new cabins in... they do love to keep people guessing when each route will get the new A380.
With all the speculation, I would think that the main consideration would most likely be which route can afford to lose 6 suites, 10 business class seats (for full J upper deck configs since it's a net gain of 18 J seats for those with the mixed J/Y upper deck) and where we can sell 90+ more Y seats (net gain of 10 Y seats for the mixed upper deck). This means the SIN-LHR-SIN rotation that is less premium cabin heavy and more Y heavy based on historical sales and load factors has the better chance of getting the new A380.