Nuget consolidate vs update
The Update tab is geared to display packages which have updates available while the consolidation tab is geared to display discrepancies between projects where both consume the same nuget package but are targeting different versions.
Sometimes the versions installed into projects within a solution can drift apart and it is necessary to consolidate the versions in use.
See https://docs.nuget.org/release-notes/nuget-3.0-preview for more information.