Why does the isoelectric range decrease as an Alanine chain grows longer?
It is because of the inter- and intramolecular associations that occur. 

In Ala, the first loss of a proton forms a two point intermolecular association which is very favourable. 
Since the equilibrium tends to the right, more $\ce{H^+}$ are liberated and the p$K_a$ is lower. 



For the second step, this stable dimeric association must be destroyed, so now the equilibrium tends to the left and the p$K_a$ is higher then it would be without this association. 



In the case of Ala-Ala-Ala, this type of dimeric association can not occur because the distance is too long. 
There is intermolecular association but with different molecules, not in pairs. 
Here the first p$K_a$ is higher than in the case of Ala and the second lower because the equilibrium is not forced so much to the associated forms. 

Finally, in the case of Ala-Ala, besides the intermolecular association between different molecules like in Ala-Ala-Ala,
there is also an intramolecular association.
This one is less stable than the association in pairs of the individual amino acids, but more than the random intermolecular ones. 
Therefore, the two p$K_a$  are in between of the ones for Ala and Ala-Ala-Ala.