Ldrawn - is the gate length you use in your schematic for design. It is "Ideal" length. It is defined as distance between Source and Drain diffusion edges.
Leff is effectivre gate length after the processing is done. It takes into account underdiffusion of LDD structure under the gate.
Leff will be changing device to device. The best you can model it with Monte Carlo which adds gaussian statistical distribution to this parameter.
If you look for some data for mismatch you can see that even two neigboring devices might have different Leff and so the matching will not be perfect.