I think it's more because it's substantive. You're actually amending the motion itself, not Mrs. Vignola's amendment. You can amend hers to do a subamendment for six hours, not eight, but you can't do a subamendment to change the original motion when we're only debating the eight hours and four meetings from Mrs. Vignola.
We just have to agree to hers first—it sounds like it—and then you can do yours.