"To miss this data" means to not have or be aware of important information that could be useful or necessary for a particular purpose or task. Full definition
Each uses slightly different data - handling techniques (such as how to account for missing data) and so each gets a slightly different (but nevertheless very similar) values. (cato.org)
A visual inspection of school names suggests zero of the schools with missing data on this indicator are magnet schools, so they are all coded as traditional public schools. (educationnext.org)
There was a major station dropout — and an increase in missing data from remaining stations — which occurred suddenly around 1990. (pjmedia.com)