For decades, the Federal Circuit operated with little oversight from the Supreme Court. For example, in the first 15 years [after 1982], the Supreme Court decided a total of only five cases from the Federal Circuit. That's a remarkably small number compared to cases the Supreme Court took from other circuits. In addition, other circuits have the disciplining effect of different judges, different circuits coming to different opinions and conclusions. The Federal Circuit acted alone.
In that time, it developed a habit of relying on rules of convenience. That is, they may have reached the right result for the particular case, but they lacked a coherent logical base. With little oversight from the Supreme Court, little dialog from other circuits, it was easy for the Federal Circuit to operate in that manner.