What can explain the yellow spots on the forewing below, shown in my image? They are not like that in the other image, nor in my field guide. Is this just a variation, or maybe a gender related difference? Could it just be the sunlight shining through the wings? It was very sunny when I shot these.

Very cool tails. For color appearance differences: In some of the books and the photo you reference there may have been flash used where the light was coming from the camera. With your shots they were backlit so the colors show through the wing and appear brighter than they would without the backlight.

Any butterfly that is backlit will show colors through the wing that you wouldn't see otherwise. I saw some Melissa Blues and when I shot from above while the sun was setting, I could see the bottom spots and colors over the bright blue of the upper wing.

Your photos are very similar to the Silver Spotted Skipper on top. If backlit they really seem like bright spots. But without the backlighting they look like glassy dull spots. Lighting makes all the difference for some.

That makes great sense to me. If flash was used in the other photo, it would also explain why that specimen seems so much lighter.