The validity of a model for multi-scale processes

IPCC WGI AR5 Summary for policy makers has been released.
http://www.climatechange2013.org/

I don't understand the implication of the final sentence in the following statements ("[Simulations] are not expected to reproduce the timing of internal variability").
In my understanding, the validity of the models and thus its future projection has been ensured because they have ability to reproduce the past internal (natural) variability of temperature.

"The observed reduction in surface warming trend over the period 1998–2012 as compared to the period 1951–2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of internal variability. "

How can we believe century-scale behavior of the models that are not expected to reproduce decadal climate cycles??  How can we believe 10 years scale behavior of a model that is not expected to explain seasonal scale behavior??  Can these timescales be really separated?   It seems that these timescales are too close to separate.

I don't hesitate to believe the 100 years-scale behavior (or equilibrium behavior) of a model that is not expected to reproduce seasonal and daily-scale behavior of the focal phenomena, because it might be possible to separate these timescales. I believe we need to develop (or learn, if it already exists) a rigorous mathematical theory for multi-scale complex dynamics to answer such a general issue of modeling. 

Otherwise, I also cannot judge if the statements from other climate scientists are reasonable or not.

For example,

MIT Climate Scientist Dr. Richard Lindzen Rips UN IPCC Report: ‘The latest IPCC report has truly sunk to level of hilarious incoherence’ — ‘It is quite amazing to see the contortions the IPCC has to go through in order to keep the international climate agenda going’