Chris, I think that what you have observed might apply to all Hybrids not just Lexus and the GS 450H.
Many, if not all, of the major manufacturers of motor cars are committing themselves to the production and sale of only Hybrid or Electric vehicles early in the next decade commencing with 2020. This will mean that demand for combustion engine only vehicles, (petrol or diesel,) will fall rapidly.Thus it makes commercial sense for manufactures to discount such vehicles that they currently stock.
Lexus, having Hybrids throughout its range will not be so affected, thus demand for their vehicles should remain steady or perhaps increase.
That`s my perception anyway and I`m sure others will have their own views.