I don't think it applies equally to everyone.. I wore a 6.5 in Jackson, I fit a 6.5 in old stock SP Teri's (those weren't a good match for my heel shape though, but that's regardless of the size of the boot, I simply needed a "deeper" heel cup), and I'm currently skating in a pair of custom Harlicks that were made to a 6.5 for my right foot, and a 6.75 for my left, and I feel like the left has just a bit too much room at the toes - though I'm not exactly convinced that boot is too large, it's just that it feels much different than I am used to, considering that it is my larger foot and I have tended to size my boots so my right foot felt secure, it meant an extra snug fit in the toes for my left... now it is probably more of a normal fit, but since it's not what I'm used to it feels weird. I probably would have been fine in a 6.5 for both feet and not felt like the left was too short.
Bottom line though, it's important to figure out what works for YOUR feet - what works someone else's sizing experience may not apply to you. We all have different feet - the shape of the toes, width of the foot, overall volume of the foot contribute a lot to how we perceive a boot to fit. Even how a fitter prefers to fit can have an impact... especially if you have a fitter who is used to working with growing feet and makes recommendations accordingly, while you are well past the age where that is really a concern and absolutely don't need any "extra" room.