Budget AWS sells you slices of CPU performance rather than leasing you hours of access to a physical chip. They gain/lose on efficiency in that context.
They sell you a second of performance on a system. There's a fair bit of variation in systems even in the same tier, but you're going to pay the same price.
And low bin chips are still going to be more power efficient than the previous generation.
And low bin chips are still going to be more power efficient than the previous generation.