How the destination changes the check
A book interior and a poster are held to different numbers. The destination you pick sets the resolution target, the bleed the printer expects, whether unembedded fonts are a blocker, and which checks do not apply at all. Changing it afterwards re-grades the same measurements; the file is not read again.
What this cannot tell you
Total ink coverage, overprint and safe margins need your printer's colour profile and a RIP, so they are reported as not checked rather than passed. No result here is a promise that a printer or a platform will accept a file.
Looking for one answer instead?
Each check also exists on its own page: the Print-Ready PDF checker, the DPI checker, bleed and trim, font embedding, page size and RGB / CMYK.