Why is my Alternative Minimum Taxable Income $38928 when I only made $21233 on my W2?

ISO is for Incentive Stock Options.

With ISO, you do not have to report income when you receive a stock option grant or when you exercise that option.

You report the taxable income only when you sell the stock. However, you do have to report that bargain element as taxable compensation for Alternative Minimum Tax (AMT) purposes in the year you exercise the options (unless you sell the stock in the same year)

 In the year that you exercise an Incentive Stock Option, the difference between the market value of the stock on the exercise date and the exercise price counts as income under the AMT rules, which can trigger an AMT liability. 

For more information, check: Alternative minimum tax