Read --delete in
man rsync carefully, emphasis mine:
This tells rsync to delete extraneous files from the receiving side (ones that aren’t on the sending side), but only for the directories that are being synchronized. You must have asked rsync to send the whole directory (e.g. "dir" or "dir/") without using a wildcard for the directory’s contents (e.g. "dir/*") since the wildcard is expanded by the shell and rsync thus gets a request to transfer individual files, not the files’ parent directory. Files that are excluded from the transfer are also excluded from being deleted unless you use the --delete-excluded option or mark the rules as only matching on the sending side (see the include/exclude modifiers in the FILTER RULES section).
Bottom line of the story is that it only does this for what you specified to be synchronized and dependent on whether you specified a wildcard or not, if you don't want that but instead want to have it happen everywhere, then you can specify the
--delete-excluded flag as well.