The routes you are flying are likely going to be a big factor. If you have been flying routes that were UA Elite heavy, but not on AA, naturally your upgrade rate is going to be higher on AA. There are almost certainly routes where you will do better on UA than AA (for example, routes out AA hostage hub DFW). You will see the same thing on DL with hostage hubs like ATL,DTW, MSP having lower upgrade rates for DL Medallions than for those who fly on other alliances out of those hubs.