For all the hoopla about being a Republican, Ike was a social liberal. In fact, it wasn't a given that he would run as an "R". Plus, at that time, the Republican party, as it was dominated by the striped-pant, Rockefeller bunch, was the liberal party. Don't forget that JFK's SC appointment (White) was one of the most conservative justices on the bench for a generation, and dissented in Roe v Wade.
So in those days, when the Republicans appointed a judge, there was no guarantee they were looking for someone conservative.
You're right. Also if you compare four Truman nominees - Vinson, Clark, Burton and Minton to five Ike's nominees - Warren, Harlan, Brennan, Whittaker and Stewart you'll see that Truman's nominees were probably more conservative than Ike's (espacially Vinson and Clark). None of Ike's five was a conservatist, even Harlan (who was very often labeled as one) was simply a moderate who refused to accept the "incorporation doctrine" (although compared to an average Warren court justice he was probably like Clarence Thomas).