You're absolutely right, Chaz. If you want to be sure, try the hat if you can. Not always possible though.The size thing is kinda funny. The first fedora I ever bought was brand new in the early 90's from the last Jewish Haberdasher on Spadina Ave in Toronto. He was in his 80's and I had my hair much longer than I do now. He spent a good 2 hours with me and I came away with a dark charcoal Biltmore Royal in a size 7⅛ Jump to a few months ago and my second fedora purchase a grey Mallory circa mid - late 60's also in a 7⅛. Now both of these hats fit the same -just a little loose.
Before I purchased the Malory I measured my head and found that it is 56.5 cm, 22¼". My next hat was a vinage Biltmore from approx the same period as the Malory, but in a size 7. Now I can wear it bUT it is definitely a 'snug-fit'.
Now this Dobbs is marked as a 7, but fits me between the two 7⅛ hats and the 7. In otherworldly where my fist 2 hats(both 7⅛) are a little loose and my third hat (7) is a little snug this Dobbs (7) is a perfect fit.
So I'm not sure what all this means other than it's best to try a hat on before you buy if at all possible, but thre you hve it.
Hat sizes should be assumed to be a ball park figure. If the hat size is in your range and you really want the hat, buy it. If it's too small, you can always tweak it with a hat jack for example, or put something behind the sweatband if it's too big.