My best guess is that IMS is also the problem in the other cases you describe. Because IMS has a person doing the analysis, it may not be as consistent as satellite data on a day to day basis. We are looking into how to make IMS more consistent.
If you can point me to examples I can look into them further.
We also added AMSR2 (in addition to SSMIS) at the same time as IMS, see
P. G. Posey, E. J. Metzger, A. J. Wallcraft, D. A. Hebert, R. A. Allard,
O. M. Smedstad, M. W. Phelps, F. Fetter, J. S. Stewart, W. N. Meier,
and S. R. Helfrich, 2015: Improving
Arctic sea ice edge forecasts by assimilating high horizontal
resolution sea ice concentration data into the US Navy's ice forecast
systems The Cryosphere
vol 9 1735-1745
Another recent publication:
D. A. Hebert, R. A. Allard, E. J. Metzger, P. G. Posey, R. H. Preller, A. J. Wallcraft, M. W. Phelps and O. M. Smedstad, 2015: Short-term
sea ice forecasting: An assessment of ice concentration and ice drift
forecasts using the U.S. Navy's Arctic Cap Nowcast/Forecast System Journal of Geophysical Research: Oceans
vol 120 doi:10.1002/2015JC011283