Family income doesn't play into EO-1, EO-2, EO-3 etc. It is an acronym for education/occupation. It is determined only for parents who had their highest education in the US (or maybe US or Canada). Each parent is assigned a EO score and the highest score is your score. No bachelors degree for either parent is EO-1 regardless of occupation. EO-2 is assigned if a parent has a bachelor's degree (or a higher level degree) and is employed in a clerical or service occupation or as skilled or unskilled labor. A parent with a bachelor's degree who is working in a professional, executive or managerial role puts you in EO-3. A parent with a master's degree working in a professional, executive or managerial role is EO-4, and one or more parents with doctoral or professional degrees working in a professional, managerial or executive position is EO-5.
It could be that last year your mother's school was incorrectly classified as a US school and she was considered to have a bachelor's degree or higher and working in a clerical role or as a laborer. She might have been reclassified this year as the graduage of a school outside the US and therefore no EO is assigned.
