He got too much in his first year of arbitration (they didn't actually go to arbitration, because they agreed on a deal before the deadline). Part of why players don't get paycuts in arbitration is those players usually suck to much to not get non-tendered. If he finishes strong this last week (and possibly the playoffs), he'll definitely get a raise if he goes to arbitration.
Why for me is a must. We don't have a left handed reliever for next year capable of doing the job, the closest is Chapman and he is not good enough and struggled at triple a. Second, Sipp is top 3 lefty reliever on FA list. Third, he has been great for 2 years with the Astros. Fourth, why lose a draft pick signing a top lefty reliever as we don t have the solution in house. Fifth, Sipp can get right handed out, meaning, if we don't get another lefty he helps going more than 1 batter as he is more than a specialist.