i wonder whether you are increasing the size of the contact patch with the road as the tires wear and effectively making it a wider tires. i can't help any more than that without seeing an actual photo.

you could roll the front and rear tire through some mustard and over some white paper to "see" the contact patch. here, i'm assuming the rear tire is more worn and that you'll see the difference. you'll have to "ride" the bike to keep appropriate pressure on the tire.

good luck.

edit: damn, beaten by a minute. i still like my mustard idea.