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By construction, the various fields covered in a mosaic have different 
coverages, and thus different
synthesized beams. Furthermore, mosaic covers wider fields, so the impact of frequency variable angular
resolution is more important than in single fields.
Yet, the mosaic is restored with a unique Clean beam. This situation leads to a mismatch between the used Clean beam and the dirty beams in some or even most of the fields. Furthermore, because Mosaics often use short spacings (see Section 8), the dirty beams often present a wide “plateau” that is not well fit by a Gaussian Clean beam. These are situations which are prone to lead to inaccurate large scale flux restoration, because the Clean beam and dirty beams area do not match.
To minimize these issues, it is recommended in IMAGER to use the FIT /JVM_FACTOR and UV_RESTORE commands that correct this mismatch to first order by re-scaling the residual by the Clean to Dirty beam ratio, the so-called JvM factor (Jorsater & van Moorsel (1995)). Only do so once you are satisfied with your deconvolution, as UV_RESTORE is often time consuming on Mosaics.