say your subreport returns it variable after CR has 'used' it. Since I don't know when CR is creating the subreport vs using the shared variable, it could be that the results of the subreport are lagging the use. Another easier example, the results of the shared variable are used in a group header, but the subreport is used in the details section or the footer section. This second example is more obvious, but sometimes I think this is what happens in other less obvious cases.
HTH