Dear Furcan,

This is a challenging question, I think the best one to answer this question is the device engineer who concepted such devices. Here, i will try to start the answer by reviewing some facts about the threshold voltage VT of the mos transistors:

- The threshold voltage decreases with channel length for short channel transistors because of charge sharing effect.

- It decreases also by increasing the VDS for short channel transistors by induced barrier lowering.

-  Since in this technology the lower threshold transistor has the longer channel length,then the threshold voltage  of the two transistor types is intentionally adjusted  by the technological parameters as ion implantation and different oxide thicknesses. 

One may conclude that the  different transistors may be required for proper circuit operation i.e. It may be requirements from circuit design.

- An other idea is that, if one uses the same VDD for both transistors, so one may make the threshold voltage of the shorter transistor larger than the longer transistor to equalize the threshold voltage under the operating voltage since its threshold voltage will be reduced with VDS such that it may approach that of the lower threshold transistor.

In general transistor designs are constrained by technology and by the circuit operations. So, you can get the cause if you  know the applications of the transistors.

I hope i shed some light on the answer of the question.

Dear Furkan,I think we will keep the more length to reduce the  gamma value of the MOSFET so that the noise offered by the trasistor will be reduced.Otherwise due to short channel effects the noise figure of the entire system will be increased.