alternative hypothesis: they just cut corners and skipped a bunch of aggregates in the road bed and shit.

In the past, you'd see a concrete road surface for widened highways - see most of 225, most of I-25 through T-Rex and I-25 between 470 and Castle Rock and you will see the road surface being concrete. But look at the more recent projects like I-70 in the mountains where the express lanes exist and I-25 north of 270/36 where the express lanes are and you will see asphalt surface. Same deal with I-25 in the gap - you see they've pained over where the express lanes will be with all the new lanes over asphalt. Yes, asphalt surface is cheaper but doesn't look as nice as a new concrete surface, which while is more expensive, lasts a lot longer than a concrete road surface.