This is from the NHC 5pm discussion...

...data from the stepped-frequency microwave radiometer instrument on board the
aircraft suggest that the surface to 700 mb flight level wind ratio
is not quite as large as we typically use...and the initial
intensity is adjusted slightly to 145 kt.

Basically what they are saying is that the surface wind has not weakened, but was merely adjusted after getting more detailed info about what is going on at the surface. A small hurricane with a 902 mb pressure may have 185+ mph wind because all the energy is concentrated in a small core, but such a huge storm like Katrina has the wind field spread out so far that the inner core wind is a bit less.