There are a lot of may's in that. The fact is no birth can be found for her in the UK and therefore you go with the obvious. Also census in the US at that time were filled in by the person coming around to the door and if the enumerator as just said where you from to the householder then he could have copied down the same info for all the family.
You are making assumptions that Evelyn's family filled the 1900 in as well what evidence is there to support that? As anyone checked the later census ( I am assuming the later ones have been released.)
Ok the 1920 is on Ancestry (I dont have sub unfortunately) as is the 1930 and the 1920 census as a Mary E ASTBURY born circa 1895 living in Montgomery OH
I would still consider US as the birth. It seems looking at the rules that the enumerator had to follow that if he could not ascertain from the householder the information then he had to answer what he believed to be true. Ill quote "92. General.—Endeavor to obtain a positive answer to each inquiry according to the instructions herein given, but if an answer to any question can not be obtained from the persons in the family or t he neighborhood, as the law requires, enter, in all cases where possible, the answer you believe to be true. In no case should "Unknown" or "Un" be entered, unless every effort to obtain the desired information has been made. "
Rob