Looking at the stats, the name [name_f]Sienna[/name_f] had a surge in popularity in [name_m]England[/name_m] & [name_m]Wales[/name_m] in 2004, probably due to [name_f]Sienna[/name_f] [name_m]Miller[/name_m] becoming famous around then. [name_f]She[/name_f] was also in the [name_f]British[/name_f] tabloids a lot at that time, and was seen as an ‘It Girl’ with an unusual name. I don’t know whether she had the same level of fame in the US?
Another factor might be that [name_f]Sierra[/name_f] and [name_f]Savannah[/name_f] are fairly similar, and both had been in the top 100 in the States for at least decade prior to 2004. Perhaps [name_f]Sienna[/name_f] just didn’t seem as interesting to American parents by then as they already knew a little [name_f]Sierra[/name_f] or [name_f]Savannah[/name_f]?