There have been valid questions as to whether the finches I recently posted on Tweeters as lesser were actually american instead, since some field marks do not match certain field guides. I am not an expert on any finch, so I won't address individual variation within Lesser Goldfinch.
I just put 2 videos on Flickr that I took in Tukwila, WA in January of this year. Both videos show both finch species side by side for comparison. In one video, the AMERICAN GOLDFINCH shows a small white patch at the base of the primaries while the LESSER GOLDFINCH apparently shows none. This is the exact opposite of what one might expect.
I urge anyone with doubts to review these 2 videos of winter finches and I believe it will be obvious that all birds of the same species are not cookie cutter copies of each other. When it comes to bird ID, there can be many shades of grey.