Did you ever think about it?
Zypper does a check (no matter if a core, update, whatever repo) of all repos for their actualisation status. If it detects that the content of a repo wasn’t updated for a long time, this could mean that the repo link or mirror is broken (or the mirror owner abandoned it …). That’s why zypper tells the user: hey, have a look there and doublecheck.
But you know the differences between a rpm based distribution and a debian on? No? You also know the differences between apt and zypper?