edit: I didn't notice it was overbust measurement! D: I thought it said underbust. Oh well, close enough if not for the overbust measurement.
As for the one on eBay, it seems it was a user written guide. There's more inaccurate ones on eBay though, my bad ._. But I saw a guide that did plus 1" for odd band sizes and no addition for even numbers before.
I can't find any size guides on eBay or Amazon, unless they're specific to a seller. Where are you finding these guides?
The one for Amazon shows up next to the size dropdown box, as "sizing info" http://www.amazon.com/gp/product/ajax-handlers/apparel-sizing-chart.html/ref=dp_sizechart_c?ie=UTF8&asin=B003L1NB9W
edit: I didn't notice it was overbust measurement! D: I thought it said underbust. Oh well, close enough if not for the overbust measurement.
As for the one on eBay, it seems it was a user written guide. There's more inaccurate ones on eBay though, my bad ._. But I saw a guide that did plus 1" for odd band sizes and no addition for even numbers before.