Home advantage and capping.

[MicroBob]

I think with past DET teams they were also possibly undervalued on Thanksgivings because they were more often than not a pretty bad team but often not quite as bad as the public would perceive them. This might have a greater impact in the line for a Thanksgiving day game which I suspect gets a little more action than your average DET game.
So you have a bit of BSP'ism potentially driving the line a bit perhaps combined with the possibility that a team going nowhere, like DET in most seasons, can still summon up some amount of inspiration late in the season for a rare national TV appearance.

Just some possible other reasons for it off the top of my head.
Your point of the super-short week impacting the away team more is certainly valid too. But I do think this year's matchup is a little different than other Thanksgiving situations in that it is unusual in recent seasons for DET to actually be considered a decent team by the public and/or still be in the playoff race.

[Thremp]

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It's good that you realize home field advantage isn't something you can express as points (especially in football), as the numbers just don't add up right. In some cases you're giving too little and in others too much. Taking this a step further and treating all games as a winning percentage and you should be able to quantify home field advantage much better than the average Joe.

That said, home field advantage isn't something I've studied all that deeply. What I do know, however, is that it might bet better to look at it as away team disadvantage. Traveling and staying in hotels in an unfamiliar place takes its toll. It isn't always about how loud the home crowd cheers (or boos)....

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A flat % adjustment is errant as well (Not that you suggest this method) since HFA is less important for teams that are huge faves or dogs already. You might actually be better off in a sport (speculation from here out) doing a cents adjustment to a line for something like baseball rather than a % adjustment.