Ct/myelogram from 2 different doctors 1st shows bulging disc no nerve impingement 2nd shows nerve impingement why did 1st not show the same as 2nd one?

Depends: If these were 2 separate ct myelograms (at different times) it is possible that the second one has a new finding. If disc pathology progresses, nerve impingement may develop over time. Alternatively, there is some subjectivity to interpretation of these studies. Some doctors have a lower treshold for calling impingement. It is more important whether this finding fits with your actual symptoms.