A White-throated Sparrow with a dark center breast spot!?

Huh? A white-throated Sparrow with a dark center spot? I have seen this sparrow before and have wondered why it has a dark center breast spot??  Unusual?  I think so!

I have seen Song Sparrows, Fox Sparrows and Tree Sparrows, all with a dark center breast spot , as was expected, but not  White-throated Sparrows, such as this one seen in bird Alley this morning.  What has been your experience?