Shuttle postponement: What am I missing?

The next Space Shuttle mission to the ISS is on track for a May 31
launch. (I asked them to speed it up a day so it would celebrate my
daughter's birthday, but you know how the government is.) But the
last mission which will service and upgrade the Hubble telescope, set
for August 28, will have to be postponed 4-5 weeks due to the need to
process two External Tanks for the shuttle and a standby rescue
mission, required on this flight because the shuttle can't reach the
ISS as a "safe haven" from the Hubble orbit.
Now, there's something here I'm missing. We've been flying shuttles
for over a quarter of a century. NASA should know by now how long it
takes to do everything and how much "fudge time" needs to be built
into schedules. The need for a major postponement because of ET
preparation baffles me. Has NASA still not learned the art of
realistic scheduling?
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