As others have noted, location isn’t really taken into account in these salary comparisons. A disproportionate percentage of graduates end up in the region their college is located. The Midwest has a lower cost of living than the Northeast or West, so schools in those more expensive regions have a bit of an edge on salary comparisons. For salaries, I recommend comparing schools within a region. There are other factors as well; schools that offer engineering also have an edge, for example.
Beyond all that, I think you might have been looking at older data, and perhaps even data restricted to those who don’t continue to graduate school. PayScale refreshed its data a couple months ago. It also provides different ranks depending on whether looking at just those that stop with bachelor’s degrees or all alumni; I personally find the “All Alumni” value more interesting since getting into a good grad program is a legitimate reason for selecting a particular college in the first place (especially for LACs). Here is what I see for the schools you mentioned using their current data, for both “Bachelor’s Only” and “All Alumni” options.
MIT: Bachelor’s Only- 1; All Alumni- 1
Colgate: 7, 22
Lafayette: 33, 41
Bucknell: 39, 20
Bates: 51, 59
Bowdoin: 45, 42
Amherst: 102, 50
Middlebury: 106, 62
Carleton: 92, 67
Denison: 98, 117
Grinnell: 256, 133
While Grinnell is still lowest from this group, the “All Alumni” figure is 123 spots higher than their “Bachelor’s Only” figure. Also, it should be noted that only the last three are from the Midwest. The others are all from the Northeast, so have a raw earnings advantage, but at the price of a higher cost of living.