The answer lies in all of the responses you have received, but one point which caused me problems hasn't been mentioned. I used featherboards (in front of the blade) but still had the same problem you mention when I was trying to cut curved boards. What was happening is that once the strip and the remainder of the board exited the blade the same pressure (from the featherboard) was no longer there and that caused the board to drag against the back of the blade causing the variation in width. I fixed the problem by using featherboards before and after the blade.

By the way I used the strips which were uneven anyways and had no real problem. Obviously that depends somewhat on the degree of thickness variation you have on each strip.

Good luck