Below are typical reasons that the cost comparison between the Health Check report and other cost reports may not line up:
The costs filtered for unattached disks on Reports > Cost > Disk
in the past month need not match the immediate monthly savings on Dashboards > Pulse > Health Check
.
The savings in general takes into account all the data available not just the the previous/current month information.
The immediate savings takes into account all the unattached disks that are present rather than just the previous/current month unattached disks.
The platform cannot back fill/line up the immediate monthly savings (as it is relative to the current time).