Well ya know Gwen and No Doubt's albums are usually slow, steady sellers.
They rarely go to #1 but they sell millions eventually. Look at what happened to L.A.M.B., Rock Steady, and Tragic Kingdom.
Those albums never went to #1, started off moderate sales, stayed on the charts for a long time and boom - sold millions
I think it'll be the same with this album. People are just starting to warm up to it