You took the pKa of the wrong group when calculating the PI. With amino acids, you'll learn that there are exceptions to many of the general trends.
PI = pH at which the species net charge is '0'.
Aspartic acid pKas:
1.9 - carboxyl
3.7 - side chain
9.6 - amino group
Think about the species at low, neutral and high pH. When pKa > pH, the molecule wins the proton.
Low pH (~1): everything is protonated
1.9 COOH
3.7 R-COOH
9.6 NH3+
--------------------------------------
+1 charge
Increase the pH so it is between the first and second groups with lowest two pKas e.g.,
pH ~3
1.9 COO-
3.7 R-COOH
9.6 NH3+
--------------------------------------
+0 charge
An easy way to determine which pKas to use your your calculation would be to order them from least to greatest and draw the species' net charge in between each pKa on a pH gradient. For example:
0---------1.9----------3.7--------9.6------------pKa
-----+1----------0---------- -1 ---------- -2 ---net charge
0----------2-----------3----------10-------------pH
(not drawn to scale)
Since neutral charge lies between the groups with pKas of 1.9 and 3.7, you'd use those numbers when calculating PI.
Let me know if I can expand or clarify.