The reason is this: up to 20% of the votes scored by ACU are basically party procedural votes or CYA votes disguised as legislation. This is not knocking the ACU or the validity of the index; it is simply stating a fact which to some extent or other is true in every index. Just as the Soros people beleive they own the Democrats, there are others such as the Chamber of Commerce or American Realtor's Association (albeit to a far lesser extent) who beleive they own the Republicans.
Accordingly, there are one-sided votes where the congressional member will go along with the party line, particularly where it won't make much of a difference to their constituency, just to hike up their rating with their key funding sources.
As proof, you can look at the ratings of Democrats turned Republicans such as former Senators Phil Gramm or Ben Campbell or Republicans turned Democrat such as Judas Jeffords and compare their before and after the switch ratings. Their views really haven't changed that much, neither has their constituency, but the funding mix of which organizations support them plus who they work with has changed, often dramatically.
The rule does not generally apply to those either on the soldidly conservative (80%) side or solidly leftist (20%) side of the index. The reason is because these people are so solidly in one camp or the other that colleagues and funding sources are unlikely to influence them. This isn't to say they won't get funding from friendly sources, particularly in close races or other sources of payback, it is simply that unfriendly sources won't send them a dime because their chances of influence are zilch. Thus, NARL will certainly contribute to Ted Kennedy (even though there is zero chance he will ever vote against them) but won't send a dime to Tom Corbun (because there is likewise zero chance he will ever vote with them).
Accordingly, Chaffees real ACU rating is 26-15 = 11%. Specter, who happens to be closer to the 80% would be 64-11 or about 53%, nearly identical to the weak sisters from Maine. Ben Nelson, on the other hand, would get a 60 + 12 = 72% rating once this adjustment is made.
Interesting stuff.