Williams have been quietly going about their business this season, the FW36 not smattered with updates at every GP as an attempt to increase performance. I'd like to classify their development as economic, finding performance from their base setup at each GP, whilst smaller updates bridge the gap. The team arrived in Singapore knowing that teams like Red Bull would take a demonstrable leap ahead of them owing to their more downforce laden chassis. Finding performance therefore for Williams has come from the team trying different approaches with their brake ducts, which of course will not only affect temperatures directly but also change how the tyre phases on longer stints.
The new configuration is looking to retain more temperature for use by the brakes, whilst lowering the core temperature of the tyre.
As with all decisions of this nature it's a performance trade off and something that can also suit one driver more than the other. Meanwhile the lap time delta between the two compounds taken to Singapore is larger than anticipated and so decisions of this nature then become even more critical for race pace.