The most likely cause of this is the fact that Republicans and Democrats had the same number of years as president during this time period. President Carter served one term and President Clinton served two. Similarly, President Bush served one term and President Reagan served two. Presidents tend to appoint judges who are from their own party. They do so in order to get judges on the bench who are likely to think in the "right" ways. Given that the two parties both held the presidency for 12 years during this time, it is not surprising that they would have appointed about the same number of federal judges.