There is nothing mystical that these two flights happen to both be numbered 181. It is just a coincidence. In fact UA's 181 has bounced around as different destinations like MCO-LAX, BOS-SFO and BOS-LAX at 11:00 and at 17:45. On the other hand, AA's 181 has been BOS-LAX for years now. You are right, however, it most likely is a problem with ATC and should be corrected on UA's end.