Thanks for the help here and thanks for advising on TT desktop.
After getting the desktop version and running a diff between before and after the ISO entry, I can see the reason why I was charged more taxes even when AMT was 0.
The reason was because even though my AMT due was 0, my AMT credits from previous years was less available. The difference between AMT and ordinary tax will dictate how much credits you can claim. The Exercised ISOs, while not pushing me into the AMT zone, gave me less of those credits.
If only TurboTax Web could actually TELL the user why taxes goes up and down, then I wouldn't have had to resort to desktop and track down why the difference occurred.
Hope this solves a mystery for someone else too!
