a) When you tram it you are only checking that the spindle axis is at 90 degrees to an imaginary plane that passes through the points on the table touched by the indicator. Try the same test at different positions along the x axis.
b) It may not have been machined flat or it may have warped after machining. Use a straight edge of known quality to check the flatness of the table.
Here's a test procedure with acceptance criteria that you may find interesting/useful.
Originally Posted by Cowbell