For medium sized creatures, this is wouldn't matter either way. 15-feet from the center of the square you are in, will reach squares 15' away from you and the coverage area in those squares would be 50%, so effectively the area of effect is 15.5-feet. For a large sized creature (or larger) casting this, then the area of effect would greatly diminish -- this is why I think they went with the wording "around you to a distance of 15-feet". Accordingly, I don't think you count the square you are in when determining the distance -- otherwise they would not have described it in these specific words.
However, I do see your point -- and while talking about "points", I suspect the point of origin rules could likely favor your interpretation. I think all area affect spells all seem to originate from a "point" and not location -- such a creature?
While I'm on the fence on the proper way to apply this, if we take this to the extreme and have a huge creature cast Spirit Guardians, the area of affect is much larger now if we apply my reasoning of "15-feet away from the area you occupy".
I would like to hear what others think about this.