You're pretty much on track with your reasoning. Less developed storms are more shallow systems and thus they are steered more by the low level wind fields. In the tropics, these wins are easterly. As a system deepens, the upper level environment has a more profound effect. Most of our numerical models strongly weight the upper level features in determining track. The models don't usually do a good job with intensity, so if a storm weakens suddenly (or strenghtens) they take a bit of time to catch up. With Irene, along with Franklin and Harvey, because these systems were relatively weak and sheared, track forecasting is more difficult.

The NHC in the 5 PM discussion points out the problems with the long range guidance with a weak storm (Irene). I think JB probably thinks the storm will remain weak and not turn NW. We shall see.

AND THERE IS A LITTLE MORE CONFIDENCE IN THIS TRACK NOW
SINCE SOME OF THE MODELS THAT WERE EASTERN OUTLIERS THIS MORNING...
SUCH AS THE UKMET AND NOGAPS... HAVE SHIFTED WESTWARD.
THEREFORE... MODELS ARE COMING INTO BETTER AGREEMENT THAT A SYSTEM
OF MODERATE TROPICAL STORM STRENGTH WILL MOVE WEST-NORTHWESTWARD
ALONG THE SOUTHERN PERIPHERY OF THE LOWER/MID LEVEL SUBTROPICAL
RIDGE. THE SLOW MOTION SHOWN AT THE END OF THE OFFICIAL FORECAST
IS INDICATIVE OF THE GREAT UNCERTAINTY LATE IN THE PERIOD...WHEN
THE SPREAD IN THE MODELS IS STILL QUITE SIGNIFICANT